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Walk it off

Running may get all the cardio cred, but walking scores just as high where it counts. Weight loss? Check. Mood boost? Check. Easy on the joints? Major check. We tapped Harley Pasternak—trainer to Jennifer Hudson and Katy Perry—to create three 40-minute calorie sizzlers. One caveat: You've really got to get moving to maximize calorie burn. "You need to move with a purpose," says Pasternak.

Next: How to optimize calorie burn