You'll need to create an exercise regimen that addresses three categories of fitness: cardiovascular health, strength, and balance. "You want to be in the best shape of your life," says Dr. Luanne Freer, director of Everest Base Camp Medical Clinic. "And you'll want to maintain that level of fitness throughout."
Cardio: Running, cycling, hiking, aerobics, or spinning classes, boxing, or jumping rope – whatever your exercise of choice, do it for a minimum of 45 minutes, three times a week. Stay between 65 and 85 percent of your max heart rate (the zone where you can feel your heart humming, but you aren't gasping for breath). Cycling and trail running are especially good, and seek out hills whenever possible. By month six, you should be able to knock off a half-marathon or century bike ride without too much trouble.
Strength Training: You want to avoid bulking up; hauling extra weight up a mountain is no fun. That means lighter weights at higher reps. Down low, do calf raises, leg curls, leg extensions, and squats; up top, do pecs, biceps, triceps, and shoulder exercises, twice a week. "You'll also need a strong core for carrying a pack," says Peter Whittaker, co-owner of and a guide at Rainier Mountaineering Inc., "so it's important to find a handful of abdominal and lower-back exercises."
Balance: Hike off-trail, where loose rocks and scree move beneath your feet. At the gym, break out the Bosu balls (the half-balls with the flat bottoms) and try the Telemark Jump: Get into a lunge position, with one foot on each ball, then leap up and switch legs in midair. It's not exactly like picking your way over a boulder field at 17,000 feet, but it's better than not practicing at all.