How to Make Your Workout Quick and Sweaty

The varying intensity of interval training burns more calories than a steady routine.

She and other trainers are also big fans of mixing aerobic and strength training to get a combined workout. Such "circuit training," in which you move through a series of vigorous exercises with little or no rest in between, "lets you knock out two things at once," says Durkin. You can set up your own circuit, perhaps with advice from a trainer, by intermingling resistance exercises with stints on a bike or treadmill. Many gyms have circuit training classes, too. Most also have shorter, 30- or 45-minute classes that are designed to incorporate both cardio and strength training; you can let the instructor do the planning for you.

Whatever you're doing, don't skip the cool-down to save time. Your body needs a gradual ramping down of exercise to return it to its normal state, and stopping on a dime can cause blood to pool in your legs and make you dizzy. But you don't need to stretch, says Ingraham; it doesn't work any better after exercise than before. So save the extra five minutes for thinking about what you'll accomplish with tomorrow's workout.