We've got some happy news that will rev up your workout routine: The moment you head out on your run, launch into your Spinning class, or start your Pilates session, the benefits of exercise kick in. Your heart rate increases, and blood is delivered to your muscles. You start burning calories for fuel. And you get an almost immediate mood boost. Regardless of age or fitness level (yup, this includes everyone from mall-walkers to marathoners), studies show that making time for exercise provides some serious mental benefits.