When interval training, you switch between short periods of intense exercise and periods of less intense exercise. Working out in this way seems to help boost speed, endurance, and muscle strength in a short amount of time. So you may get more fit in less time than your standard workout. Want to add it to your current routine? Just pick up the pace during certain parts of your workout. For instance, run for 1 minute on the treadmill and then walk for 4 minutes. Repeat this 5 times.
Interval training is safe for most people. But talk to your doctor before you try it out. And make sure to talk with your doctor before starting any new fitness routine.