Walking for exercise with a new twist: carry a backpack weighted with bags of sand to increase walking's conditioning effects. Authors Bar-Khama, former chief physical education officer of the Israel Defense Forces, Schoenfeld, an internist, and Shuman, an American journalist, praise walking as an ""all-purpose"" exercise, safer and easier than calisthenics or running. (Long hikes are what keep the crack Israel Defense Forces in shape.) But they argue that the benefits of walking, like those of any exercise, diminish unless you work harder and harder at it. Instead of longer and longer walks, they advise carrying ever-heavier backpacks during a half-hour walk, five days a week. A fitness test (climbing up and down a chair) enables you to determine in which of five fitness levels you belong, and a six-week program of progressively speedier walking and heavier ""weight-loading"" follows. But, sound as the regimen undoubtedly is, anyone with the patience and discipline to calibrate distances walked, fill bags with sand, and spend half an hour a day walking is probably exercising regularly already. The particular appeal here is to admirers of Israeli muscle.