Health-conscious doctors today advise regular exercise to build fitness of body and mind. To quote an authoritative new home medical manual -- "The Columbia College of Physicians and Surgeons Complete Home Medical Guide" (Crown, $39.95) -- exercise "tends to lower blood pressure, helps control weight, increases level of high-density lipoproteins (the 'good' cholesterol) in the blood and may help control diabetes," as well as improving muscle tone and flexibility and enhancing energy and well-being. Also: "Some people who exercise find that it helps them control stress, and many smokers give up the habit after they start exercising."

All exercise helps, but only aerobic exercise -- exercise that requires increased oxygen intake -- makes the heart stronger. Some pointers from the Columbia guide:

*"Cardiovascular conditioning occurs when the program involves sustained effort for a duration of at least 30 minutes per day, at a frequency of at least three days per week, beginning at an intensity of approximately 70 percent and increasing to 85 percent of the maximum heart rate."

*"Maximum heart rate, or the maximum an individual's pulse rate can attain, is calculated by subtracting his or her age from 220. So, for example, a 40-year-old has a maximum heart rate of 180. Exercise should . . . raise the pulse to between 70 and 85 percent of this number, or 126 to 153 beats per minute. Less exercise gives the heart little conditioning; anything above 85 percent is dangerous."

*"Individuals can determine whether they are within their target zones by taking their pulse immediately after exercise . . . Place two or three fingers lightly over the carotid artery, located on the left and right sides of the Adam's apple, count the pulse for 10 seconds, and multiply by six. If the pulse is below the target zone, the rate of exercise should be increased; if above, it should be reduced. Pulse rate should be checked once a week during the first three months of exercise and periodically thereafter."

*"For those who have not exercised regularly, it may take several months to raise the rate above 70 percent, and it is wise not to do this in any way but gradually . . . The optimal frequency for those beginning an exercise program is two to four times a week. In fact, three times a week is really all that is necessary at any stage."

*"Good aerobic exercises include brisk walking, running or jogging, bicycling, swimming, skating and jumping rope. Non-aerobic exercises include bowling, golf, weight lifting, doubles tennis and volleyball since the level of activity is not sustained enough to be aerobic."