Since shaking up her fitness routine, Moira Jrolf is spending less time exercising yet getting better results.

She used to focus just on long periods of sustained physical activity - such as an hour-long run or 1.5 hours of nonstop lap swimming - but over the last couple of years she's been incorporating short-burst interval training into her fitness regimen. She does sprints at the track or in the pool, for instance, with rest periods in between. But those brief breaks are more than made up for by the high intensity of her sprints.

"It definitely is good to mix it up because you feel like you're really pushing your limits with the interval workouts," says Jrolf, 41, of the Boston area.

And, says the busy mother of four young children, "On the days I do my interval training, the workout is shorter, so I put in less actual time working out."

When going full speed ahead, one just can't sustain the activity for as long. "On the days I do the interval training, I save about 20 to 30 minutes per workout," she says. So instead of lap swimming for an hour and a half at a stretch, she takes an hour-long interval-based swim class. And one of her three running days each week involves 30 to 40 minutes at the track doing sprints rather than a slower, hour-long run.

The approach has paid off. Since Jrolf started interval training, she says she's become more toned, faster and much less bored while exercising. Last April, she completed her first marathon.