Even for runners who never make the transition to more sophisticated workouts, easing into speedwork will lead to more enjoyable running.

The body responds to a calorie deficit by slowing down the metabolism and burning muscle tissue. That leads to weakness, sluggishness, slow times. In girls, it can also result in cessation of menstrual periods, which in turn leads to loss of bone density and frequent stress fractures.

Fartlek, or speed play, is variable-pace running that emphasizes creativity. During a 30-minute run, choose objects to run to - telephone poles, trees, buildings, other runners, whatever. Make choices that mark off different distances, so your pickups vary in length from 15 to 90 seconds, and modify your pace to match the distance.

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