High Intensity Interval Training involves alternating between very intense bouts of exercise and low intensity exercise. For example, sprinting for 30 seconds, then walking for 60 seconds is high intensity interval training. You increase your metabolism and burn more calories for up to 24 hours after interval training. Why?  Your muscles require additional oxygen in order to work at this intensity level therefore forcing your body to utilize body fat more efficiently. Don't add more time to your aerobic exercise each week, simply add power.