Don't have hours a day to dedicate to physical activity? You're not alone. Luckily, you only need a half hour to really boost your activity, metabolism and energy levels. “Interval training helps boost your fitness level without having to spend more time. You only need a 30 minute workout to do circuit training, where you alternate high and low intensity activity,” says exercise physiologist Heller. For example, alternate short bursts of jogging and brisk walking. If jogging is too much, you can add periods of faster walking during a leisurely walk. An added bonus: These workouts tend to go faster because you're mixing it up and challenging yourself every couple of minutes. Try to do interval training several times a week.