What happens to your body as you age?
Your body starts to naturally decline in function when you hit your 30s. Your muscles lose strength. Your aerobic capacity decreases. It's harder for you to balance. You become less coordinated, less agile and have slower reaction times. You are less flexible, and you begin to feel the effects if you have poor alignment.

To deal with these issues, you need to adopt an exercise plan that targets these vulnerabilities. "The aging process starts at 30," says Jeff Young, exercise physiologist and fitness manager for Beth Israel Medical Center's Continuum Center for Health and Healing. "The older you get, the more important it is to start a strengthening program. By the time you get to be about 50, not only is it important to start strengthening, but you need therapeutic exercises." Fixing poor alignment and staying flexible are key as you age; poor alignment can stress your joints, and repetitive activity makes you more susceptible to injury.

The big three do's
If you want to stay as active and fit as possible as you age, consider walking, some type of more rigorous cardiovascular exercise, and weight training. "These three components have been repeatedly shown to make a difference in a person's rate of aging," says Michael Roizen, M.D., chief wellness officer for the Cleveland Clinic and co-author of "The RealAge Workout" (HarperCollins, 2006) and The New York Times best-seller "You" series of books, including "You: Staying Young" (Free Press, 2007).

Roizen suggests walking 110 minutes a day, which is about 10,000 steps. He suggests 21 minutes of cardiovascular exercise three times a week at 80 percent of your age-adjusted heart rate (both men and women use the same formula: 220 minus your calendar age) for 20 minutes, with one more minute at a maximum effort, as long as your doctor approves. For weight training, he suggests at least 10 minutes three times a week, being sure to involve your eight muscle groups: upper back, lower back, upper leg extensors, upper leg flexors, lower leg, upper chest, core and upper arm muscle groups (biceps and triceps). Voila. The basics of fitness for longevity. But there's more.

It's important to focus on your strength fibers, rather than your endurance fibers, when lifting weights. "Your strength fibers are the first to go," says Young. This means that rather than using light weights with more repetitions, to increase the endurance of your muscles you instead need to lift heavier weights with fewer repetitions. Young says to start with slightly lighter weights and higher reps, such as in the 12- to 14-repetition range, and progress toward heavier weights and fewer repetitions, such as in the six- to eight-repetition range. But remember: Heavy is relative. Never push yourself to the point where you could be risking injury.

For your cardiovascular exercise, choose something that offers lateral movement -- in addition to backward and forward movement. "You want to maintain your speed, stamina, power and agility," says Young. "Choose something that has your body moving in all different directions and offers some sort of aerobic component."

Go for variety so your muscles can adjust and react to different types of uses. Ball sports, such as basketball and soccer, and racquet sports, such as tennis, are good choices. Other healthy cardio exercise choices are dancing, hiking and swimming. Choose something you like to do and you'll find it that much easier to stick with.

Two potential don'ts
Long-distance running and golf are two sports that garner red flags from our experts. Running is especially hard on your body because it wears out your parts. "When you do an activity more than two hours at a time, you begin to do more damage than benefit," says Roizen. Especially damage to your joints. "Being physically fit does not protect against bone and joint injuries," says Roizen. "The two things that destroy joints are malalignment and overuse." Joint injuries occur more commonly in people who do the same activity for more than two hours, says Roizen.

And that's where golf comes in. If you want to get out there and walk the course, that's healthy. It's the swinging of clubs that puts you at risk, especially if your swing is poorly aligned.