You don't have to be a distance runner or athlete to benefit from interval training. Whatever your level of fitness, age or choice of exercise, interval training energizes, slims you down and can boost your fitness to astonishing levels. Best of all, you won't pound the pavement for hours on end or spend your life (and your bankroll) in the gym. Twenty minutes, three or four times a week, is all it takes to get in the best shape of your life.