The New Rules of Fitness

My life in fitness: In my 20s, I ran, took aerobics classes and played tennis. Back then it was all about looking good and having fun. In my 30s, I stepped up the running to relieve stress and traded aerobics for ab work to get my stomach back in shape after having two kids.

Now I'm 46, and my priorities have changed again. All that pounding on pavement and too many two-handed backhands have done a serious number on my knees, hips, and lower back. And I needed a new routine to do a better job of fighting gravity. (I simply refuse to sag.) Without giving up the activities I love, I had to update my approach. I'm still running, but I do it on a treadmill or track so it's less pounding. Between runs, I'm exploring kickboxing, Spinning, Pilates, and yoga. I haven't looked back.

Follow my lead: This step-by-step guide will help you continue to reap all the stress-beating, endorphin-inducing, disease-fighting, and age-defying benefits you can only get from a really good sweat.

Whether you're an exercise veteran or a newbie, your fitness needs change once you hit 40. Here's how to fine-tune your workouts.

Do Cardio at All Speeds

Have you been working out at the same speed? It's time to branch out. Varying the intensity of your cardio keeps you seeing results. There are four basic levels: A moderate zone (your breathing is slightly deeper than at rest); a steady-state zone (your breathing is deeper and you're going at a good clip); an aerobic zone (deeper-still breathing and the pace is challenging); and a threshold zone (your breathing is sharp and your muscles are working to fatigue). You can move through these different levels within the same workout, or during different sessions or with different activities.

Try It: Twice a week, do interval training: alternating bursts of sprinting with bouts of moderate pace to recover. "By making your heart rate repeatedly spike, you hike up the calorie burn," says Wayne Westcott, fitness research director of the Southshore YMCA in Quincy, Massachusetts.

Circuit training -- hopping among several cardio activities within one workout -- also propels you past your status quo. "Be sure to choose activities that use different muscle groups. So, for example, run, then do the elliptical trainer, and then cycle. By resting some muscles while using others, you don't fatigue as fast, so you can keep working harder," says Westcott.

Incorporate speed play into your sweat session and your everyday life -- speedwalk to a meeting or walk up the stairs in slow-mo (slowing down means you use more muscles and less momentum). Mix up your fitness routine every month or so by rotating your favorites or trying something completely new. Note to newbies: When you present your body with the challenge of a new activity, it expends more energy. That translates into quicker toning and weight loss.

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