Better to exercise at high intensity for a short time or low intensity for a long time?

That all depends upon your fitness goals. Do you want to be able to complete your 45-minute walk to work without arriving in a huff? A long, low-intensity workout is what should be on your training schedule. Is playing competitive soccer your goal? Opt for the four-mile, 30-minute run. "A vigorous workout will let you sustain other vigorous workouts," explains Barbara Hansen, Ph.D., director of the Obesity and Diabetes Research Center at the University of Maryland Medical School. "Moderate biking for an hour and a half won't help you sustain a competitive tennis match, but it will gradually improve your ability to do other moderate forms of exercise."