High-Intensity Interval Training (HIIT) is challenging. It takes your cardio workout to another level, as you push your pace out of your comfort zone. You’ll work up a sweat fast, working at a very intense level and then backing off for a slower recovery period, followed by another round of high intensity. That strategy can save you time: You don't have to work out as long as you would if you were keeping a steady pace. You’ll lose weight, build muscle, and boost your metabolism. Plus there’s a post-workout bonus: Your body will burn calories for about 2 hours after you exercise.