A simple, sound walking and jogging program--with a few calisthenics and some stretching to give folks over 40 a carefully graded cardiovascular, strength, and flexibility workout. DeVries, director of the physiology-of-exercise lab at U.S.C.'s Andrus Gerontology Center, recommends a three-hour-a-week plan that requires short bursts of low-intensity activity over a relatively long period of time (instead of the highintensity exertion of programs for younger athletes). To determine how much walking or jogging should be done, pulse-counting is provided for (with clear instructions); the four calisthenic and nine stretching exercises are also adjusted to fitness level. Why exercise? It ""lops years off chronological ages"" by increasing heart muscle strength; has positive effects on blood vessels; improves breathing capacity; burns fat; keeps joints mobile and builds up bone; and ""works wonders for mind as well as body."" With a well-planned, low-risk program like this, those are benefits worth trying for.