(FORTUNE Magazine) – If I told you I could prescribe something that would prevent heart disease, diabetes, and obesity, and make you feel great, you'd be at the drugstore in a heartbeat. But hold on. I'm talking about exercise. The list of conditions it prevents and treats--depression, colon cancer, hypertension, and osteoporosis, to name a few--borders on the awesome.

In a study of almost 22,000 men--some fat, some thin, some just right--the fit ones lived longer. But here's the remarkable thing: The men who were fat and fit lived longer than those who were thin and not fit. In another study--this one of Harvard alumni--the exercisers lived an average of one to two years longer. And those who began exercising later in life still lived longer.

With results like that, you'd expect everyone to work out. Trouble is, only about 20% of us exercise regularly. And almost one-quarter of the U.S. population is completely sedentary.

Aerobic exercise--jogging, biking, swimming--strengthens the heart, improves fitness, and burns calories like a furnace. If cardiovascular fitness is your goal, try to work out three to five times a week, 20 to 45 minutes a session, and get your heart going at a good clip. For a rough range of the beats per minute you should shoot for, subtract your age from 220 and multiply the answer by 0.7 and 0.8.

Resistance training (weight lifting) builds strength and muscle mass. Because muscles burn calories much faster than fat does, adding muscle increases your metabolic rate. Studies show that even one set of 12 repetitions of an exercise that fatigues a muscle will increase muscle mass and strength.

Stretching--the third component in a well-rounded exercise program--is great for flexibility, helps prevent lower-back strain, and is good rehabilitation for many muscle and joint problems. Don't bounce; slowly work your way into the stretch until you feel tension, then hold it for 20 to 30 seconds. Also, contrary to old beliefs, the best time to stretch is after exercising, when muscles are warm.

If you're over 40 and just starting to exercise, or if you have any risk factors for heart disease, consider a treadmill test. It can spot problems, give you feedback on your level of fitness, and help determine a more accurate target heart rate than the generally accepted formula.

Let me leave you with a few warnings. Being a weekend warrior can increase the risk of injury. Consistency is more important than going for a personal best every other Saturday. As a colleague of mine puts it, strains and stress fractures often result from the terrible toos: exercising too much, too fast, too hard, too long. The goal is to improve your health, not make it worse.

FOR MORE INFORMATION ON THIS TOPIC: Go to mayoclinic.com. Mayo Clinic offers Executive Health programs at Mayo Clinics in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.