Men and women of the boomer generation now have a book about exercise catered to their specific needs and circumstances. A recent New York Times article on the elderly and exercise proclaims. Encouraged by doctors to continue to exercise three to five times a week for their health, a legion of running, swimming and biking elders are flouting the conventional limits of the middle-aged body's abilities, and filling the nation's operating rooms and orthopedists' offices in the process.