dswithers said:Sprint 100 yards, turn around and walk slowly back 100 yards to the start, then turn around and repeat until you have done 8 circuits, should take less than 20 minutes total. For an alternate try sprinting up hill 70 yards or up stairs or stadium steps... The sprint should be an all out, 100% effort.
Many studies have shown this type of exercise is more effective for heart health, growth hormone release, overall fitness, weight loss, etc than 60 minutes of steady running!