Today, if national polls are to be believed, millions of Americans exercise at least three times a week, doing everything from jogging and cycling to aerobic dancing and weight training. But the image of a nation of exercise fanatics may be an illusion. Public health experts say that only about 10 percent of adult Americans exercise regularly. In essence, say the experts, the fitness boom that began 20 years ago has run out of steam. While the debate continues over how many adult Americans actually exercise, there is more universal concern about the nation's children. Numerous studies indicate U.S. youngsters are in generally poor shape, though one influential researcher believes that while some children are quite unfit, youth fitness isn't a public health problem.