ENDURANCE IS THE FOUNDATION of The Shape of Your Life because this workout plan is about going places—the top of Mount Washington, three weeks down the Back River, the finish line of 24 Hours of Moab. Technically, endurance is a combination of efficiency (lean body mass), physiology (a dense network of mitochondria that produces energy in the muscles), genes (a high percentage of slow-twitch muscle fibers), plumbing (an efficient heart capable of moving more blood per pump), and strength in those areas that help transfer force between the upper and lower body (the hips, lower back, abdominals, and other core muscles).

How do you build endurance? First, you need to put in steady, sustained periods of activity—running, biking, swimming, rowing—at moderate intensity to build your muscular and aerobic base. "If you're always running out of gas after an hour, that can be indicative of not enough foundation," says Ray Browning, seven-time Ironman winner and coauthor of Serious Training for Endurance Athletes. "In some sports, like cross-country skiing and cycling, it can be easy to always work at too high an intensity and never develop your low-intensity base of endurance." But base-building workouts in tandem with intensity training can bring about significant leaps in aerobic efficiency.

Brace yourself—here comes the lesson from Exercise Physiology 101. As intensity increases from moderate to high to very high (think jogging, running, sprinting), you compromise your body's ability to produce the energy needed to power muscle contraction. You can sustain a very high level of effort for brief periods because you've crossed your lactate threshold. (Lactate is a byproduct of lactic acid that can't be burned as fuel.) At this point, you shift from aerobic (oxygen-aided) energy production to anaerobic (non-oxygen-aided) energy production, and lactic acid is pouring into your muscles in such large amounts—hence the burn—that they shut down. With proper conditioning, you can push this threshold back. Proof? Watch Lance Armstrong, a lactate-threshold-training devotee, pedal away from the peleton on a long climb. As his competition falls behind, legs searing, Armstrong's able to keep spinning—and he has yet to cross his lactate threshold.

To lift your LT, you first need to find out where it is—easily done, thanks to the development of wireless heart-rate monitors—and run an interval now and then close to that number. You can estimate your LT using a simple calculation that approximates your maximum heart rate (see "The Prime Rate," last page), the highest number of times your chest ham can go flippity-flip in one minute. Your MHR isn't a direct indication of how fit you are, and it will vary from sport to sport. But this number is invaluable because the body marshals its different energy systems at various percentages of maximum heart rate with remarkable consistency. At 70 percent of MHR, it uses oxygen to burn fat; at 85 percent it begins breaking down muscles for fuel; and at 90 percent it burns carbs exclusively. Not many athletes can surpass 90 percent of MHR without hitting the lactate wall, when muscle contraction—and therefore you—grind to a halt. Depending on your fitness level, your own LT lives somewhere between the 75 and 90 percent mark.

The first month of The Shape of Your Life dedicates three days a week to aerobic and LT training. These sessions will repeatedly push your LT by way of intensity drills—what you've probably come to know as intervals. At the end of each month, you'll gauge your progress with an easy time trial. As you find yourself running a mile faster at the same heat rate, you'll know you have a bigger engine and a higher tolerance for lactic acid. Congrats. You now have more than a running routine; you have endurance.