Endurance interval training
Credit: Photograph by Justin Steele

One of the most influential running studies in recent years, published in the 'Journal of Applied Physiology,' found that runners who did half the mileage at higher intensity had meaningful improvements in their blood pressure, cholesterol, and V02 max. They also had major improvements in both 1,500-meter and 5K time trials. That's further proof why nearly every elite runner incorporates some variation on this kind of training. "It's a very simple idea, really," says Greg McMillan, the author of the new book 'You (Only Faster)'. "You're taking a rest interval so you can do more work at a higher intensity."

The Routine
After a half-mile warm-up, run 30 seconds at 30 percent effort (a fairly easy jog); 20 seconds at 60 percent (breathing hard to the point where talking becomes difficult); and then 10 seconds at 90–100 percent (a breathless, all-out sprint). Repeat five times, continuously, rest two minutes, then repeat the whole cycle at least two more times.