Nearly all runners do their slow runs too fast, and their fast runs too slow." Ken Mierke says. "So they're just training their bodies to burn sugar, which is the last thing a distance runner wants. You've got enough fat stored to run to California, so the more you train your body to burn fat instead of sugar, the longer your limited sugar tank is going to last." -The way to activate your fat-burning furnace is by staying below your aerobic threshold--your hard-breathing point--during your endurance runs.