"The secret of enjoying exercise is rather simple," claims aerobic guru Dr. Kenneth Cooper. "You should pick an aerobic activity that really interests you . . . schedule a definite program for pursuing that activity and commit yourself to following it for at least six weeks."

Because people who drop out of a program usually do so in the first week, "most who stay with it for a month or more," he says, "begin to taste some of the tantalizing benefits available."

Cooper recommends a minimum of three--and preferably four--approximately 40-minute sessions a week. His four-step plan for tailoring a personal program:

1. Have a thorough medical examination, including a stress test. Particularly important to those 30 and older and "essential if you're over 40."

2. Determine your target heart rate. To get the maximum cardiovascular benefits from aerobic activity (called the "training effect"), you must maintain a sufficiently high heart rate during exercise. First, take your resting heart rate by monitoring your pulse at the wrist or over your heart (not at the neck since some studies claim it's less accurate) for 15 seconds and multiply that figure by 4 to get the number of beats per minute. To calculate your Predicted Maximum Heart Rate (PMHR):

Men, 205 minus one-half your age; women, 220 minus your age. Calculate 80 percent of your PMHR. If your heart rate exceeds this figure for a minimum of 20 minutes, three times per week, you will get an aerobic training effect. (Since the heart rate of highly conditioned people may drop significantly in the time it takes to monitor the pulse, they should add 10 percent to their post-exercise pulse rate.)

3. Choose a basic aerobic exercise. The top five--in order of descending exercise value--are cross-country skiing, swimming, running (faster than 9 miles per minute) or jogging (slower than 9 miles per minute), outdoor cycling and walking. (Unless your stationary bike is one of the new kinds requiring arm movement, "you'll put out considerably more effort to get the same benefit as you will with outdoor cycling.")

Other options: roller skating, aerobic dancing, racquet sports, mini-trampoline, skipping rope.

4. Embark on a regular program. First warm up for several minutes to stretch and ready the muscles and heart for exercise. Then move into the aerobic phase. Follow exercise by at least five minutes of cool-down movement. Finally, spend at least 10 minutes on calisthenics and/or weight-training activities to build muscles and increase flexibility.