In a recent study, subjects who spent just 20 minutes mixing sprints with jogging lost three times the fat off their legs and seat in 15 weeks, compared to those who jogged steadily for 40 minutes. Intervals may trigger fat-mobilizing hormones and they amp your cardio capacity so your future runs will actually be easier. If you can't run, you can accomplish the same goal on a stationary bike, stair stepper or water aerobics.