New federal guidelines announced in January 2005 advise adults to exercise moderately to vigorously for about an hour most days just to prevent gaining weight as they age. Those who’ve lost weight need even more activity, up to 90 minutes of moderate exercise, to keep the pounds off. Only the relatively few who have no trouble controlling their weight can meet their exercise quota in as little as a half-hour, the guidelines indicate. (For people seeking to lose weight, the report merely recommended increased physical activity and reduced caloric intake.)

There are several ways to make those new recommendations less daunting. Start slowly and gradually work up to longer sessions. And try squeezing small chunks of exercise into your daily routine. (See Did You Know?, below.) A particularly effective method is called interval training, where you alternate bursts of more-intense activity with stretches of easier exercise.

The interval method--applicable to virtually any aerobic activity, and an option on most exercise machines--avoids long periods of strenuous exercise. That helps prevent lactic acid, a waste product of muscle activity, from reaching levels that can make exercise painful and exhausting. So you can reap greater benefits without feeling significantly more tired.

For example, suppose you usually walk at a moderate 3.5 miles per hour. Picking up the pace to 4.5 mph during several short segments that add up to half of your next walk would let you burn about 30 percent more calories. Similar benefits accrue when more-fit exercisers switch between, say, brisk walking and jogging. Those differences, multiplied over weeks and months, can lead to far more pounds lost as well as other health improvements.

Moreover, the interval method lets you satisfy the new requirements without actually exercising longer, since you can ratchet up your workouts just by increasing their intensity. For example, that 30 percent increase in calories burned using the interval method is roughly the equivalent of exercising 30 percent longer at the original pace. So if your usual walks took 45 minutes, you can meet the one-hour requirement just by adding peppier intervals, without lengthening your sessions.

Another benefit of interval training: It can help you surmount an exercise plateau, rather than trying to intensify your entire workout.

For most people, the best way to choose the appropriate intensity of each interval is to rate your exertion on a scale of 0 (no activity) to 10 (almost maximum effort). Beginners should generally alternate between either 3 (moderate effort) or 4 (somewhat strong), and 5 (strong) or 6 (between strong and very strong). Experienced exercisers can increase the intensity slightly. Note: See your physician first if you’ve been physically inactive or have a chronic disease or elevated disease risk.