Start at the goal line of a football field. Sprint all out to the 10-yard line, and then back to the goal line. Repeat, this time running to the 20 and back. Then to the 30 and back and so on for the length of the field. Rest as needed to keep up a strong pace; the faster you run, the more you'll build power. Work up to 15 to 20 minutes of sprinting with only short rests.