The way to shape up fast is no secret. Weight training will sculpt and build lean muscle, which (unlike fat) burns calories on its own; cardio (brisk walking, biking, jogging, etc.) will shed the fat hiding those defined muscles. I'm a cardio fan from way back (I love how it feels to get my heart rate up), but who wants to slog away on a treadmill in a dull gym, going nowhere fast (or slow, as the case may be)? Not me! Especially when you can spend less time and get better results.
The secret is intervals. Study subjects who spent just 20 minutes mixing sprints with jogging lost three times the fat off their legs and butt in 15 weeks, compared to those who jogged steadily for 40 minutes, research from the University of New South Wales in Sydney finds. Intervals may spark fat-mobilizing hormones, and they amp your cardio capacity so your future runs will actually feel easier.
I like relaxing runs, but if you do the same slog every day, you can suffer from what's sometimes called "postman's syndrome"—named for those who do the same walk every day and yet their bodies never change. Your muscles become so efficient at a movement over time that if you do it day after day and eat the same way, you never overtax your system and burn more calories to lose weight.
The answer is to mix up your cardio either by changing your workout around or adding speedy bursts—meaning intervals. Intervals can be fun if you approach them right. Each one represents a mini goal—finish the 1-minute sprint, complete the 2-minute cooldown, and so on. I feel more pumped up doing this and never get bored. Check out Self.com's workout archives for tons of cool cardio plans.