It's not that I hate the gym, exactly. I just hate what it represents, the way its very existence amounts to a challenge unmet – a reminder that there are fit people in the world, and this is where they go to do the things that fit people do. Those aren't my people: athletic types with a surplus of self-regard, and the vanity to spend precious hours sculpting their deltoids or hoofing mindlessly on a treadmill. I sort of pity them. Even the ones who aren't ripped – those lean, dutiful health obsessives – are suspiciously preoccupied with self-preservation. So narrow is their pursuit of longevity that they overlook the soul and poetry of life while they're checking their heart rates and calculating their BMI.

So what if life's poetry accumulates to form a ham-size band of flesh around your middle? So what if it leaves you flabby and weak where you really want to be steely and solid? So be it. At least you've not mistaken fitness for something important in life.

I hadn't exactly let myself go. I'd flirted with decent shape before – decades of semi-regular running, a span of consistent yoga practice – but it was never done in concert with good nutrition and rarely involved real sacrifice. (I was one of the few yogis who would be tucking into a lamb chop a half-hour after rolling up my mat.) As I saw it, working out gave me license to consume what I wanted, like an insurance policy against slob-dom, but one whose protective qualities had diminished. The few times when I did make a concerted effort, I'd stick with it only long enough to be surprised at how easily the initial weight came off. Then I'd pat myself on the back and resume the old habits. Such was the level of my commitment.

At 6-foot-1, weight hovering between 198 and 203 – those are estimates; I only weighed myself at the doctor's office – I was, according to the accepted calculations of some unrealistic, faraway authority (in this case, the World Health Organization), considered "overweight." I guess I was a little soft, covered in a smooth, lifestyle-induced layer I'd accepted as the status quo. There was muscle under there, still bubbling beneath the surface, waiting to be called up to a boil, but lately it had cooled to a gentle simmer. And what I knew about male physiology was that, due to falling testosterone, decline was inevitable. At some point I'd lose the ability to build muscle. How long did I have before the flame went out completely?

I didn't want to find out.

There would never be a better time to get in shape. I was turning 45 in an America that was awash in training opportunities, nutritional advice, and whey protein shakes. Fitness had finally trended away from bulk-building and toward a new respect for the kind of lean strength found in rock climbers, not bodyguards. And if I were to make some progress, there'd be more ways to measure it than ever before. The kinds of tests and scans and performance-based diagnostic equipment that were once confined to Olympic training camps and East German preschools were now available to a regular schmo like me.

RELATED: Everything You Know About Fitness Is a Lie

I wanted to take advantage of it. I would learn and sweat and get tested, and finally get in the kind of shape that would let me pursue any sport, fitness class, and physical challenge without fear of injury or abject humiliation.

OK, I'd settle for no more injuries. I'd had a few of those lately – more than in all my previous forty-some years. Minor knee surgery (torn meniscus) had left me with few exercise options – I couldn't run or jump rope, and riding my ill-fitting bike inflamed the tendinitis in my elbow. So I wound up at a local gym, pursuing a haphazard workout based solely on vanity, familiarity, and avoidance of the exercises I hated the most. I did crunches, flys, curls, bench presses. I even did those fussy little push-downs that work no part of your body other than the triceps. I ended up trashing my shoulder doing a set of irrationally exuberant push-ups on one of those giant green balls that people sometimes use as office chairs. At the same time, the tendinitis in my elbow returned like a wasp you had tried and failed to swat. More discouraging, my once flexible left hamstring had gone brittle on me, hardening into a cranky, tender band that made me walk like Walter Matthau.

All my efforts toward fitness had somehow pushed actual fitness further away. I'd gotten used to lying in bed, a triangle of discomfort – left knee, right elbow, left shoulder – mummy-like until the pain in my hamstring woke me up and reminded me that I had to take a leak.

But the injuries didn't hurt as much as the implication: I'd arrived at midlife flabby, feeble, and shying away from most physical challenges, even recreational ones. I was bowing out of invitations I wouldn't have thought twice about accepting a few years ago: Tough Mudders, CrossFit, bowling. (Yes, bowling.) When did I get so pathetic? I needed a professional to set myself straight. So I walked into a gym and asked for help. I tried to keep my goals humble and my vanity in check. I wanted to build muscle and lose fat, of course, but more fundamentally, I wanted to get balanced and aligned and strong, from the inside out. To reinforce the function-over-form prerogative I started the personal renovation in mid-September, the official end of the shirtless season.

Before I did any renovating, I needed to find out what kind of raw materials I was dealing with. Any man looking for a thorough assessment of his physical state would be hard-pressed to find a place better equipped than the Iris Cantor Men's Health Center at the Weill Cornell Medical College in Manhattan. It was conceived as a place where a man can see several specialists in one visit to get a full picture of his health. Once I arrived, urologist Dr. Steven Kaplan, the center's director, and his colleagues set about quantifying the metabolic me.

Their blood and urine tests yielded more information than a 44-year-old in reasonable health needs to know, including calculations of red blood cell distribution, iron saturation, and glucose, which were all fairly unremarkable. There's even a number to quantify the strength of my urinary stream. (Race horse, apparently.) There were more typical tests, too – ones that measured things like cholesterol levels and testosterone, which was surprisingly low, given my general susceptibility to road rage, distractibility, and other forms of male idiocy. But as both doctors remind me, as a healthy patient with no troubling symptoms (my breasts aren't sore, and I'm experiencing no sexual dysfunction), this was a minor deficiency I'd never be aware of otherwise.

The complete profile of my physical state is still only a sketch, and I'm already regretting my decision to sit for a portrait. Things can only get worse, I think as I remove my pants and lie flat on the Lunar iDXA body scan, a state-of-the-art GE diagnostic tool that will slowly scan my body and measure, with X-ray-enabled precision, what portion is fat and what portion is muscle. The six-minute scan produces a full-color rendering of the fat that's gathered around my middle and along my hips. Then it goes one better by also producing a multipage document calculating the body-fat density of every surface, every limb, each side of my torso, my hips, everything. It tells me that my more dominant right arm is 23.5 percent fat (hardly gunlike), while my left is even chubbier, at 26.2 percent. My android region around the waist, an area the literature cruelly describes as "fat often associated with apple-shape bodies," is 27.8 percent while the gynoid region around my hips, "fat associated with pear-shape bodies," is 26.2 percent. I have no idea what the difference is, but I wonder how hard I'll have to work before the body scan stops comparing me to fruit.

A few days later, Dr. Hugo calls me with test results and points out a few areas of concern: One is my LDL, that sinister cholesterol, which he has tallied at 110 mg/dL. It's not alarmingly high by any stretch, but it's "above optimal." The other is more exotic: an enzyme found in the liver, called AST, which Dr. Hugo tells me is high enough (93 IU/L) to indicate some "unhappiness." I've seen this movie before. "You might want to cut back on the alcohol," he suggests gently. And really, who am I to deny my liver a happy life?

One of the first things I discover under the tutelage of my Equinox trainers is that I've not paid nearly enough attention to my core. That vaguely defined network of muscles – roughly from below the chest to the tops of the knees – is nothing short of almighty. It is every bit as essential to functional fitness as its name implies. My trainers – Equinox training manager Vural Vural (who goes by the more succinct "V") and trainer David Juhn – have a deep and abiding respect for the core.

Equinox subjects new training recruits to a test. The Functional Movement Screen (the FMS) was developed by physical therapist-strength trainer Gray Cook, who sought out a uniform way to assess fitness beyond strength and flexibility. The FMS [see the Assessment: How Well Can You Move?, p. 98] involves few movements you typically see at a gym, and nothing you'd glimpse on Muscle Beach. Instead, it features simple stepping actions, basic squats, some rotated lunges, and one seemingly impossible motion that asks you to get on your hands and knees and then lift your right leg and right arm off the floor simultaneously.

None of this goes well. Of a possible 21, I score a 12. The number matters less than the dysfunction it represents: According to V, I am a casualty of the modern office environment – hunched, misaligned, and, having let some important muscle chains go fallow. In anatomical terms, I lack the all-important core competency, and my adductors – a group of muscles in the thigh – are dominant, as illustrated by the way my knees flare during the squat. Most important, I suffer from what V calls "glute amnesia," which not only contributes to my knee issues but also is kryptonite to core competency. "You spend so much time in a seated position at the office that you forget how to engage your glutes" [see Working-Stiff Workout, next page].

He has another name for the condition – "dumb ass" – but no matter what we call it, it's clear I'll get nowhere if my glutes and abdominal wall don't form a chain of musculature that is able to transfer power from my lower girdle (hips) to my upper girdle (shoulders), and from side to side. It's a key transaction that makes everything else possible. "Your core should be firing first on any activity you do," he tells me. "It's imperative to everything you do." After the first week – I train with each of the trainers once a week – I question whether I'm even sitting or climbing stairs correctly. Actually, I know the answer: I'm not.

For the next few weeks, no one asks me to do a crunch or a curl or a bench press. Most of the work we do uses my own body weight, an occasional medicine ball to squeeze between my knees (glute activation!), and some machines to improve rotational mobility in my torso. Vural keeps me on a strict diet of core activation. So while hunks of iron are being hoisted and swung around me, I lie on my back thrusting my hips in the air while V pokes at my butt and abs. I am an island of Jazzercise in an ocean of 300. V and I rarely wander in the more gratifyingly macho gym material – that's more the realm of David, who has me do pull-ups and such, but only after squats and low-to-high cable pulls for my torso. They are both sticklers for form and speak more like orthopedists than trainers.

It takes just a couple of weeks for me to shed some pounds and feel a slight shift in the age-old metabolic tug of war between fat and muscle. The results are mostly interior and invisible, but there is a new solidity forming beneath the surface. Walking feels different, with more awareness in my hips and better posture, more balance. One morning I reach down to tie my shoes and realize that I've lifted my foot instead. And I do this without falling over.

The twice-weekly sessions are challenging, with sets often ending in fatigue, but they're hardly sadistic. Though praise is hard to come by, there's little of the macho shaming that makes high school football such a misery. After one less-than-exhausting workout, I ask V when we're ready to increase the load. "You're not ready," he explains. "Let us build you up first," he tells me, "then we'll destroy you."

My new life is regimented but manageable. There is more restraint, less frivolity, and plenty of the sprouted-grain bread that evokes Boulder in the 1970s. My diet has been stripped of hot-fudge sundaes, Tuesday-night gimlets, and sides of bacon. Three weeks into the new regime, I open the fridge, scanning the shelves for some blueberries, some leftover cod, or a swallow of white wine, when I discover a trio of tall, shiny cans of my favorite beer standing neglected behind the almond milk. How long had I left them just standing there? I can't even remember the last time I had fries. I've become one of those people who eat those coarse, whole-grain crackers that resemble, with depressing accuracy, corrugated cardboard. I smear them with natural peanut butter, pile them high with tuna and tapenade, anything to bring flavor to the packing material that now somehow qualifies as a snack.

Thankfully, I've not had to count calories or weigh my food. I don't have a diet to follow. As it turns out, eating the things you're pretty sure you're supposed to eat and not eating the things you're pretty sure you're not supposed to eat is the diet. Another way to put it: I start thinking of flour as sugar and try to cut way down on sugar.

The one surprise involves breakfast. Joy Pape, a nutritionist recommended by the Cornell Weill Center, tosses out the zero-fat Greek yogurt (what I thought was a virtuous breakfast) because it lacks protein. She suggests an actual protein shake, which I come to love even if it's texturally impotent, lacking crunch, crust, and brown-sugar topping. I have a fairly sensible, wheat-free lunch most days. Dinner varies, but I try to eat a lot of fish, nothing more than a reasonable amount of starch, and lots of vegetables. I might have a cookie or some fruit for dessert, but I'm not standing in the pantry with an open bag of chocolate-chunk cookies, wolfing indiscriminately. The Greek yogurt I used to eat for breakfast has been elevated to "treat" status. I top it with blueberries and a paltry amount of homemade granola and call it dessert, that thing you eat before you fall asleep to the sound of your rumbling stomach.

My primary dietary weapon is the awareness that I now bring to everything I consume, particularly anything that includes bitters or cocktail onions. Even wine is suspect. The era of the bottomless glass of riesling is apparently over. For what must be the first time, I am conscious of the calories in every blessed, sweaty glass. I learned how to not get terribly plastered years ago, but now I was drinking for my health. It isn't fun or carefree or soulful in any sense, but it isn't difficult either. I get used to not having enough to drink.

After more than a month, my trainers and I are still focused on the unsexy, the functional, the invisible. This is fine by me, but it's still a blow to my inner meathead. V explains his medicine/candy philosophy: "I give some of what you want in terms of seeing results – burning fat, building muscle – while also giving you what you need." He doesn't check the size of my pecs or the definition in my arms, and when he steps back to assess me, it's my posture he's checking, and it's improving. "Pretty good," he says, poking me in the gut. "Tighten up."

So I do my lunges, my squats, my planks. Rotational stability and core competency are my twin objectives, though my posterior chain – the network of muscles that connects along the back of the body – also needs attention. V tells me it's the key to sitting up straight and engaging the rhomboids, the muscle situated between the spine and the shoulder blades, rather than pressing my shoulders into service and injuring them further. So I dutifully stick to back-opening pulls rather than pec-popping presses. When I do get my hands on some iron, I don't exactly lift it: I hold dumbbells by my side while I step on and off a bench press, or swing kettle bells to enhance my core. I hold them and do lunges and haul them up and down the gym corridor. Only occasionally do I press them overhead, but as part of a squat sequence that makes few claims on getting jacked.

When I'm not training with David or V, I do cardio: Two or three times a week, I strap on a heart rate monitor and climb on an Arc Trainer. As per their instructions, I don't hold the rail, relying on core and glute activation (what else?) to keep upright as I put myself through the paces. After a few minutes' warm-up, I do intervals [see the Cardio Project, p. 104], cranking up the level and incline for a minute on, two minutes off, all the while fiendishly gazing at the heart rate monitor on my wrist for a suitable spike. To me, this is the height of self-obsession, the exact fitness-minded myopia for which I've indicted others, and I love it. I soon become restless and dissatisfied when I don't hit 173 or higher during a workout.

In some small ways at least, I am becoming the gym type I envied and looked down upon: preoccupied with self-improvement, a little too pleased by my own progress. But it's fitness, not plastic surgery. I've earned the gains I've made, and making them isn't turning me into a preening narcissist. It's just making me stronger.

One day, I'm lying on my back between a rack of kettle bells and a bench press, squeezing a ball between my knees while V pokes me in the ass. "Tighten those glutes," he hisses as he tries to slap the ball loose. I'm gritting my teeth, intent on keeping the ball where it is, when I first spot the guy in the middle distance: a scrawny septuagenarian at the pull-up bar. Joe Lieberman in gym shorts. And I watch from the corner of my now sweat-stung eye as he hops up, gets himself comfortable, and knocks out a set of pull-ups like he's opening the trunk to his Lexus or bouncing his grandson on his knee. Just like that. Eight, 10, a dozen. I lose count. That's when I make up my mind: That guy, that skinny bastard who probably lost his virginity listening to Sinatra – I wanna be that guy.

Two months in and two days shy of my 45th birthday, I subject myself to the scale, the calipers (fat pinchers, I call them), and the numerical reality of it all. I wonder if the progress I feel I've made will be backed up with actual figures. The news is good: 186 pounds. BMI of 19 percent, down seven full percentage points. Thirteen pounds of fat.

I look at the scale, the chunky metal part settling on territory and on numbers not used to calculate my weight since the 1980s. I'd take a picture with my phone, but I'm afraid that holding it might tip me up to 187.No one ever tells you that you could stand to lose 10 or 15 pounds, until you go ahead and lose it. Then, walk into a party full of people you've known forever and haven't seen in months, and you'll know. "Oh, my God," a friend's petite wife says, looking up at what was once a double chin. "You lost weight. What, like, 30 pounds?" Guy friends embed their admiration in emasculating put-downs. "Oh, can I get my newly svelte friend a drink? I think we have some Tab in the fridge." However it's said, it is said, and it's striking how much heavier you must have looked to the world.

After close to three months, it's time to assess my real progress. Some changes are more quantifiable than others: I have lost between 12 and 14 pounds, brought my BMI down from 26 percent to 19 percent, and my VO2 Max up from 44.5 to 49.1. Dr. Hugo calls with a new set of test results: My LDL cholesterol has dropped from 110 mg/dL to 78 mg/dL, and the enzyme he was worried about has dropped from 93 to 37 IU/L, transporting my liver to a placid state.

But most important, I'm leaner, lighter, and more efficient. The difference is tangible and visible. Me minus seven percent, with more sinew than flab and the contours resembling actual musculature. (It turns out that a few weeks spent stepping on and off a bench press while holding dumbbells does more to harden your legs than 20 years of running.) But the real test, the one I most want to ace, is the FMS, that humbling survey I was subjected to at the start of all this.

It's not the cakewalk I hoped it would be: My inline lunge lacks the rock-solid stability I thought I'd earned, my squat results in a milder but still unsightly flaring of the knees, and I'm not much closer to achieving the core-reliant feat of simultaneously raising a right arm and leg from all fours. Still, I've improved core competency, opened up my chest while strengthening my back, and increased my ground stability. I score a 16 out of 21, up from my pathetic 12. "I've only scored a 20," Vural confides, in a way he thinks I might find comforting.

The truest test of functionality is actual function, of course, and I have to admit that every aspect of my physical life has improved. I sit up straighter and run with more purpose, less pain, and heightened muscular awareness. Yoga is no longer a spectacle of flailing limbs; my planks are steady and filled with purpose, and I'm strong and flexible where I was once feeble. I play tennis for the first time in months and find a torsional force generated in my middle. It starts in the hamstrings, moves up through the core, and transfers into a whiplike motion across my torso. No, really. It results in the kind of velocity and control that any tennis player can appreciate, and no one needs more than I do.

Few people would look at me now and think of ropey forearms, a rippling back, or a powerful chest. My shirtless visage in the locker room inspires neither envy nor fear. But I'm smaller and tighter than I imagined I would ever be, there are veins in my arms I didn't think existed, and the muscle beneath the surface is tuned up and ready to go.

In many ways, I'm on my way to the best shape I've been in since I was 16, in those few perfect years after you've grown out of boyhood but before you grow into the bad habits that will drag you toward certain decay – before you develop big ideas about the guy on the treadmill and the way you'll live your life. The clock has not stopped, but it's slowed just long enough to change course. At 45, that's all you can ask for.

The Test
Six minutes with GE's Lunar iDXA Body Scan tells you everything you ever wanted to know about your body composition but were afraid to ask. The scan lays bare, in stark and specific color detail, where the fat is. Hint: It's yellow. The scan on the left reveals a BMI of 26 percent with extra fat in the android (or waist) area. The scan on the right shows the same body, three months later and 13 pounds lighter, with a BMI of 19 percent.

The Assessment: How Well Can You Move?
The Functional Movement Screen, or FMS, gauges and assesses human movement patterns: simply, how well you move. On my first FMS, I scored a 12. "A 12 is not horrendous," physical therapist Gray Cook assures me, and he should know – he's the guy who invented the FMS. "That's the average at the NFL Combine. Now a lot of those guys scoring that poorly run unbelievably fast or hit unbelievably hard," – things I certainly don't – "but they're one-trick-ponies. We'll see marathoners get single digits on the FMS. They can run a marathon, but they'd die trying to climb a rock wall." Cook's research in the 1980s concluded that strength and flexibility weren't reliable predictors of injury, that people in good shape were getting injured at the same rate as those who weren't – so he developed the FMS, which measures symmetry, core stability, and motor control, which Cook says are huge factors in predicting injury. "Do you shift when you squat? Can you lie on your back and lift one leg as high as the other? Can you lunge with control? If you have asymmetries like these, they get compounded by mountain biking or downhill skiing or whatever. These aren't a complex series of movements. They only ask you to show an average range of motion. I think most martial arts instructors or Pilates instructors would look at a movement screen and say, 'Shit. If you can't do that, what are you standing in front of me for?'"

Three Crazy Exercises to Melt the Fat Away
Occasionally my trainers subject me to high-intensity bursts to burn fat. One day, David instructs me to leap from squatting position to full extension with arms overhead, laterally along the gym's long hallway. He calls these Frog Leaps, and they induce a lung-burning, heart-pounding speechlessness I haven't experienced since suicide drills in high school. Another day, David makes me crawl along the hallway on all fours, which is challenge enough, but then he makes me do it backward, leaving me flailing and shaking and humble all over again. Another day, V puts me on a treadmill with its power off, tells me to hold onto the bar, and run all out for 30 seconds. I gasp through the 60-second breaks in between, stop at five, and start coughing. "That's lactic coughing," V tells me, smiling. "It's what happens when you get lactic-acid buildup in your bloodstream from lack of oxygen."

The Cardio Project
VO2 Max is a technical way of describing cardiovascular capacity. It's a measurement of the amount of oxygen your body can utilize during an intense period of exertion. It's considered a reliable marker of cardio vascular fitness. So Ann Marie Miller, head coach at the Performance Center at Sports Center at Chelsea Piers, Manhattan's fitness megaplex, administers the test, putting me in a mask, plugging me into the machine, and putting me through my paces on a stationary bike. Her assessment: weak. So Miller assigns me a steady diet of intervals, knowing that repeating a cycle – ramping up my heart and recovering, ramping up and recovering – will reap greater benefits than maintaining a steady level. The recovery periods allow my body to repair and adapt to the higher levels. She shows me how to sync my Garmin GPS/heart rate monitor to a website (enewleaf.com) where I can upload my workouts, then leaves me to my own devices. It makes me wonder if I should go back and drop a few sprints into every slow-and-steady run I've taken since 1988. Three months later, she tests me again and this time, the news is better. Ann Marie gets visibly excited when the results begin to form on her laptop. "All right!" she exclaims. "Great job." My VO2 Max has improved, from 44.5 to 49.1 ml/kg/min. It's not an astounding leap in my eyes, but one she says she's "thrilled by," particularly since I did no bike training to get there.