To keep track of your effort during intervals,Use a heart-rate monitor like polar's GPS-enabled RS800G3 ($500; polarusa.com). Find a flat course and run as fast as you can for a half-mile (two laps on a standard track) to a mile. Check your moniter at the endthis figure is your max heart rate.
Part One: Cardio
A good cardio base is essential if you want to ski or ride until the bullwheel's last turn. Start with three 30-minute sessions per week, and mix up your workouts between running, biking, and swimming, as outlined here. But don't feel like you need to do all three each week. Mix it up whenever possible, and do what you enjoy most.
TRAIL RUNNING: Besides building endurance in your calves and the quadriceps-hamstring-glute muscle complex, the great benefit of trail running is eccentric strengthening. "When you're running downhill, you're slowing your body weightjust as you do while skiing bumps," says Hodel. "This strengthens your muscles by lengthening them."
MOUNTAIN BIKING: Riding also pumps up your reactive timingthe ability to respond quickly to terrain changes and rapidly approaching objects, like trees. And, like trail running, mountain biking improves balance. "It's a great cross-training activity for trail runners," says Hodel, "because it requires more explosive power from the legs but has less joint and bone impact."
SWIMMING: This complements your lower-body-intensive running and biking workouts by strengthening, stretching, and lengthening arm, chest, and back muscles. Swim with as smooth and stable a freestyle stroke as possible. Again, shoot for 30 minutes a session when you start.
THE GOAL: Increase each session's duration by 10 percent each week for the first two monthsby the end of month two, your workouts should reach an hour. If you started with three workouts per week, try to up it to four or five. As the third month begins, make one or two of your weekly sessions an interval workout. This will push your anaerobic threshold, helping your body operate at high intensity for longer stretches. Start with 30 seconds of running or biking at 8590 percent of your max heart rate (see below), followed by two minutes of moderate-intensity activity (4050 percent of max heart rate), for ten cycles. For each of the next four weeks, extend the length of your intervals and rest periods by 30 seconds apiece. Don't do any other workouts on your interval days.