Get on the rowing machine. "It literally works the entire body," says Valencia. Sprint on it for 90 seconds (approximately 250 yards), then recover for a minute. Do as many 250-yard sprints in 10 minutes as possible. Runner-up: Alternating between a full-on sprint and jog on the treadmill or outdoors. Do three one-minute sprints at 8.3, 8.5, and 9 miles per hour with two-minute jogs in between.