Sick of the Gym? Try This.

Intense five- to 20-minute intervals that increase lactate threshold (for less leg burn), boost VO2 max (more engine power), and beef up your ability to handle back-to-back, bell-to-bell ski days with workouts you can power through on your lunch hour. It's called CrossFit, and according to founder Greg Glassman, it will increase your cardio response and train your body to work as a strong, unified structure. This month, we asked alpinist, ski mountaineer, and CrossFit trainer Mark Twight to take us through the program that helped him drop 10 pounds, double his squat, and dust most of the race-division competition in the 2004 Wasatch Powderkeg randonnée race. The best part: His workouts last 20 minutes or less.