You spot them on the trail every morning. They pass you and nod slightly, a whiff of recognition before they are gone, but you'll pass them again, because you're doing two loops of the same 10-mile route around Town Lake, in steamy, triple-digit Austin. You see them in the pool, their gear – kickboard, fins, paddles, pull buoy – identical to yours. If you are swimming in the next lane, you look at the workout they've propped against their water bottle to see if their total yardage exceeds yours. If it does, you put in another thousand yards. You pass them on the highway riding their tricked-out featherweight bike, too focused on their cadence or heart rate to lift a gloved finger from their aero bar in recognition. You recognize but only acknowledge them if they are fitter than you – and then resolve to be in better shape by the end of whatever season it is. It's always a season, there is always something you're training for, because "training" is what you do. Women will leave you because you cannot stop training, and you will eventually have trouble getting out of bed, but that will not stop you from lacing up your shoes or diving into the pool or clicking into your pedals because you do what you do for your health, the kind of health they call good.

Somehow you are in your mid-fifties. Every year you lose more fitness, but, amazingly, through sheer discipline and relentless dedication and hours of training, you weigh not only less than you did in high school but just a couple of pounds more than you did after you came down with Lyme disease in your early thirties and went, in the space of a month, from 160 to 132. What you remember, if someone mentions Lyme disease, is not the scary weight loss but how you found a doctor who said it was OK for you to swim laps even though the sickness made you so exhausted you went to bed earlier than your three-year-old daughter. Like an addict with a nose for an M.D. who has a happy hand with the Vicodin scripts, you can always find a doc who will sign off on your training plan.

One day a girlfriend leaves you a voice message before noon (all calls go to voice mail before noon, because morning is when you train). She says – it becomes her joke; she takes great delight in saying this because she knows how it irritates you – "I guess you're out exercising." You don't fucking exercise. You train. What is the difference? Exercise is aerobics. It's a spin class. Pilates. Training is necessary because there is a goal. The goal is a race. You have signed up for the race not because you love to race – though an entire wall of your attic is papered with race numbers – but because it allows you to justify the endless hours you spend training. You spent the money, so you put in the miles.

RELATED: Are You Addicted to Exercise?

That girlfriend goes away and there is another and she says to you when she breaks up with you over the phone, at the end of the summer you spent training for a 50-mile trail race, "You were never around. You were always running." You get back together with that girlfriend because you love her and tell her you aren't really training that hard these days, but she moves into your house for a few weeks and you have a fight about how she never vacuums the dog hair off the rug. She leaves you the next day, saying, "I don't know why I have to vacuum it; all you seem to do is train all day."

The one thing you do not rationalize or ever fully explain to anyone who asks is not only the fact that you like pain and suffering but why you need it. The truth is that you would rather develop hypothermia swimming from Alcatraz Island to Fisherman's Wharf, or grow so delirious during a 24-hour trail race that you hallucinate Sarah Palin wearing a fanny pack standing trailside, than sit with the depression so deep and persistent inside you that it feels unfixable. You know you are not the only person in the world born with a genetic disposition toward despondency, but you tell yourself that you are different from the rest. You can mostly outrun it.

You are lucky – very lucky – to have landed a good job in academia. Summers off, Christmas break, a flexible schedule. You have always been a writer who writes in the morning, but at some point you realize that you haven't written anything in several months. By the time you get done training, it's afternoon and you can barely string a sentence together. You must choose between training and writing and, after eight books, writing is not the reward it once was.

At a conference in Tucson, you are on a panel with six other authors and someone asks about the writing process, and you make a fool of yourself talking in running lingo, comparing "junk mileage" to the ridiculous idea that a person should crank out a set number of words per day. "You need to be as strategic in your daily writing regimen as you are when training for a race," you say to the audience of mostly older women penning mysteries and romance novels. You get overly animated and lean forward into the microphone and say, "For instance, some days I might do a long run and others speed work and then a tempo run and then there are days I focus only on my form," and you go on and on until the moderator cuts you off. When the panel is over, one of the other writers waits with you outside the motel for the shuttle to the airport. "I would have thought you'd be running back to Texas," he says.

Good God, the thousands of dollars you spend. Running shorts, running shirts, gels, energy bars, race registration, a new Speedo every six weeks (the chlorine eats away at the elastic until they sag). Even a 10-year-old bike requires constant maintenance – tune-ups, wheel trueing – and things you can do at home that require you to purchase chain oil, special brushes to clean the cassettes, special bike wash.

You find a girlfriend who understands the training because you pitch it to her like this: Either I take medication or I train, and the latter seems much better for my body than antidepressants and mood stabilizers. But then she comes with you to the pool and sits in the bleachers and reads a novel. You happen to look up at her once and she's staring at the high rafters above her, as if she's trying to remember why she is sitting in the bleachers of a natatorium. A month later, she figures it out and leaves.

When the endorphins that supply the so-called "runner's high" fail to prevent your plunge into what you have come to call "the ravine" – a sadness into which, no matter how hard you train, you sometimes slide – you make an appointment with a therapist. The therapist asks you a few questions about your eating and training habits, then proceeds to tell you stories about the many patients she has worked with who, due to overtraining, have had hip- and knee-replacement surgeries in their thirties. She mentions former obsessive athletes – the sort foolish enough to run during ice storms – who now walk with canes. She insists you see a cardiologist and a dietitian. You submit to an EKG, and the cardiologist says you have the heart of a 30-year-old. The dietitian is not so encouraging. "I have plenty of experience working with endurance athletes," she says, "and frankly, some of them don't get better." When you ask her what she means by "don't get better," she mentions patients whose overtraining has led to anorexia, malnutrition, heart attacks, and death. You leave her office a little shaken. In the elevator, you realize that it's not dying you fear but pushing yourself so hard that you will one day have to give up running and resort to wearing a floatie belt in order to kick around the deep end of a swimming pool.

"Throw out your scales," the dietitian says. You toss them in the Dumpster and then immediately drive to Target and purchase a state-of-the-art pair that measures not only weight but also BMI and body-fat percentage. You have sworn, in the past, that you would never lie to a doctor – it makes no sense, given how much they charge – but when the dietitian asks you if you threw out your scales, you think, Hell, my insurance will cover it; all I'm responsible for is the co-pay. "Yeah," you say, "I dumped those suckers in the trash, just like you told me to."

It has been going on for years. Longer than you realize. You find a present your daughter got you for Christmas when she was six: a cardboard clock with movable hands, which reads gone running! "Move the hands to let me know when you'll be back," she says as you unwrap the present, and you think, That's sweet, but I'll never use this. She knows when she wakes up and I am gone that I won't be back until she's starving, unable to find anything to eat for breakfast but Clif bars and bananas. By the time your daughter is 25, she is designing calendars with icons at the bottom – a runner, a swimmer, a cyclist – for total monthly mileage, which makes it easier for you to add up your yearly numbers, so that at parties you can tell people how many miles you ran last year and they can nod and go off to pour themselves another glass of wine.

You have never been much for gadgetry, but over the years you become more of a gearhead. The GPS watch, you tell everyone, has changed your life. You can now monitor your pace, your distance, how many calories you burn. You realize that the latter is wildly inaccurate, yet you take great delight in telling people that you burned 4,000 calories during a 50K race when it was 8 degrees out. All this talk about a watch. You listen to yourself and it sounds like a far-off, desperate, nutty you, because the real you would never go on about a watch.

What is it that scares you so much that you'd lie to a doctor or, far worse, skip the annual family beach trip, missing out on the only chance to see your spread-across-the-country siblings, because the beach they chose is lousy for cycling? What would happen if you took the advice of your daughter, your parents, and your ex-girlfriends, and just chilled the hell out? Taking a day off is not going to turn you into the middle-aged guy in the mall wearing Sansabelt slacks. You are not going to join the table of dudes at the sports bar wearing oversize football jerseys to disguise their girth.

But sometimes you are forced to take days off. Sometimes you're stuck in airplanes, or sick (which happens often because your immune system, from all the overtraining, is as compromised as a newborn's). Those are the days when you realize how happy training makes you, because when it is taken away from you, you are so insanely anxious and miserable, it's like someone died.

And so you sign up for another race, even though your IT band is giving you fits and the elevation map on the race website resembles the dramatic peaks and valleys of your heart-of-a-30-year-old EKG. Yes, you will limp for days afterward and, yes, you might eventually be forced to take more than a week off, but this race will be worth it, even though at this point you know you are not going to age out of it with grace. You're not going to age out of it at all.