No disrespect to the 45,000 runners tackling the ING New York City Marathon this week, but you may be working too hard. If your goal is to build strength, run faster, or burn calories more efficiently, “you absolutely don’t have to run 25 miles a week,” says clinical exercise physiologist Angela Corcoran. There’s a quicker and, some say, better way to torment yourself: high-intensity interval-training (HIIT), half-hour-long workouts completed two to four times a week that cycle between peaks and valleys of hard and medium-effort exercise. Compared with running, say, five miles four times a week, a HIIT workout helps avoid ailments like aching kneecaps and plantar fasciitis, shooting pains on the soles of the feet. It helps you shed more weight, too, thanks to afterburn—long after a HIIT regimen, your body is still torching calories, whereas “a typical jog hardly has any afterburn,” says Chelsea Piers running coach Brady Crain. Scientists are still puzzling out exactly why HIIT appears so effective, but much of it has to do with how it recruits muscles. “When we do moderate-intensity exercise, a population of muscle fibers basically remain dormant,” says Martin Gibala, a professor of kinesiology at McMaster University in Ontario. “When you do interval training and hit the muscles very hard, they all get activated.” The one caveat about the short-and-fast run, says Gibala: “There’s no free lunch. If you want to get away with less time, you have to go hard.” The good news: The agony lasts only a half-hour.