He suggests "pyramid sets" -- 25 yards fast, then 25 yards easy. Increase distance: 50 fast, 50 easy; 100 fast, 100 easy, resting 10 to 20 seconds between each set.
"You're increasing your metabolism more that way than by just going at 70 percent for a longer time," Patten says. "Any interval I think is beneficial: heart rate high, steady aerobics, then zap it up again. In 20 minutes, you can ... really feel spent."
Every mile of swimming is equal to about four miles of running, he says. People run two or three miles and think it's "a good-enough workout. Swimmers get a mind set of 'only 1,000 meters' being not any good. But if you do it as intervals, that's a good workout."
The expert: Kristin Moses, co-owner of Body Bar fitness studio in Travis Walk.
The advice: Start with a three- to four-minute warm-up by doing 30 seconds of jumping jacks, 30 seconds of high knees, 30 of glute kicks, 30 of jumping jacks.
"That gets you moving, gets the oxygen moving," she says.
From there, go into a series of squats and lunges: 15 squats at moderate tempo, 15 lunges with your right leg and 15 with your left.
"Come to the floor and do 20 to 25 full or modified push-ups. You're moving from your legs, big-muscle groups, to upper-body big muscles and chest, which builds your heart rate and puts you into fat-burning zones."
Next, turn over and sit, hands on the floor behind you. Raise your hips off the floor for 20 to 25 triceps dips.
Repeat the sequence, and this time "pick up your pace, get your butt moving," Moses says. "Do the leg sequence again. You're building stamina."
The third go-round, make it a sprint: "Do jumping jacks as fast as you can, high knees as fast. You're starting to spike."
Finish with ab work, she says. Come to the floor and do a plank for 30 seconds to a minute. From there, turn onto your right side and do a side plank. Repeat with the left side.
"Voila!" she says. "You're done, and you're in amazing shape."
Fast workout advice
Mix it up. If you do 20 minutes of cardio one day, do strength training the next time.
Make the most of your time. "Some people do 100 biceps curls with the same little dumbbell," says Vingren, co-director of the Applied Physiology Laboratory at the University of North Texas. "That's not really resistance training. The benefits aren't there. Plus, you're wasting your time. It takes forever and is not efficient."
Gear your workout to your lifestyle. "What else are you doing that could be counted toward a workout?" Leverette says. For people on their feet all day, he suggests doing a 20-minute workout focusing on flexibility or strength training. For someone doing a lot of lifting, use the time for cardio.