Short, high-intensity sessions are tops for boosting endurance— that's the time-saving news from a study published in Medicine & Science in Sports & Exercise. After one month, people who did 30-minute interval-cycling workouts (going at an all-out effort and then recovering at an easier intensity) three days a week improved their V02 max twice as much as those who biked at a steady, moderate pace for an hour four times a week. "A higher V02 max allows your heart to pump more blood with every beat," says David Swain, Ph.D., the study’s author. "That means you can exercise longer and go faster with less effort." To rev up your workout, try alternating between a rate of perceived exertion (RPE*) of 9 and 5, moving at the higher intensity as long as possible before easing up. It's a shortcut to a slimmer, fitter you.