Mixing short bursts of effort into whatever you're already doing — walking, running, swimming, using an elliptical trainer, or cycling — is a great way to enhance your workouts without making them longer. Consider research published last year in the Journal of Applied Physiology. During the study, eight women in their 20s rode stationary bikes, alternating between very difficult four-minute energy bursts and two minutes of recovery. After doing ten sets of intervals every other day for two weeks, subjects' fat-burning capacity improved by 36 percent and cardiovascular fitness rose by 13 percent.

“What surprised us most was that we saw such a quick improvement in aerobic capacity,” says Jason Talanian, PhD, a researcher at the University of Guelph in Ontario, where the study was performed. Based on the results, Talanian recommends completing one or two interval sessions per week, working up to longer intervals. For instance, after a five-minute warm-up, speed up for two minutes, then work at a normal pace for the same amount of time; vary between hard and easy for about ten minutes. Gradually increase interval length by a minute a week until intervals are four minutes long. You should notice improvement in cardiovascular stamina after just a couple of weeks.