Although vigorous, heart-pumping activities melt calories the fastest, they'll do little for your waistline if you have to stop after only a few minutes. Instead, you want to work at an intensity you can keep up long enough to burn 200 to 300 calories, 5 to 6 times a week — the amount research shows is the most effective for weight loss. (That's about 30 to 60 minutes* depending on your fitness level and activity.
To find your perfect zone, use how you feel as a guide, aiming for a level 7 (on a 1-to-10 scale). You should be slightly breathless but able to speak in short sentences. To increase your calorie burn and get fit faster, add a few high-intensity intervals. Work ten 1-minute bursts, going as fast as you can, into your normal routine; you'll torch nearly 20% more calories during a half-hour workout.