But that doesn't mean that lifting weights is a waste of time. Resistance training has been shown to improve health risk factors such as blood pressure, cholesterol, bone mass and insulin resistance.

And it keeps you strong, probably decreasing your injury risk while you're racking up all those cardio minutes. And it's especially important to lift weights if you're dieting, since resistance training can help preserve lean body mass, most of which is muscle, that's normally lost when dieting alone.

If you're not used to doing cardio, start slowly for 10 to 15 minutes at a time and work your way up to faster, longer and/or harder sessions gradually.

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Martica Heaner, Ph.D., M.A., M.Ed., is a Manhattan-based exercise physiologist and nutritionist, and an award-winning fitness instructor and health writer. She has a Ph.D. in behavioral nutrition and physical activity from Columbia University, and is also a NASM-certified personal trainer. She has written hundreds of articles for publications such as Self , Health , Prevention , The New York Times and others. Martica is the author of eight books, including her latest,