Bet you didn't know that the Samburu Tribesmen in East Africa drink up to ten quarts of milk a day, eat no vegetables and only a little meat. They don't worry about cholesterol, get fat when they age, or have high blood pressure, and some of them live to be ninety. Their secret? They run a lot, and if you do, too, following the day-by-day fitness programs set up here by author Tulloh (a biologist with a distinctly anthropological bent), you can reap all the benefits of primitive cultures, and still keep your indoor plumbing. Gauge your fitness with the Harvard Step Test (pulse-taking plus exercise for five minutes), then proceed with one of the several six- to ten-week exercise plans outlined here for civilized folk in varying degrees of terrible shape.