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Q: How do I know if I'm pushing myself hard enough when I exercise?

A: The easiest way to gauge your exercise intensity is to take the "talk test." Try having a conversation. If you can blabber endlessly, like a teenager on a cellphone, you're slacking off; crank up the pace or your heart and lungs won't even know you've worked out. On the other hand, if you're so winded that you can barely utter your name, dial down your effort. Otherwise, you may end up with a slew of aches and pains – possibly even injuries – and you're likely to burn out on exercise quickly.

For a more precise gauge of your effort, rate yourself on the RPE (Rate of Perceived Exertion) scale of 1 to 10 — one being extremely easy, like sitting on the couch watching "60 Minutes," 10 being so maxed out that you feel you're going to collapse momentarily. Aim to land between a five and an eight.

An even more precise method is to monitor your heart rate, the number of times your heart beats per minute. You can do this by taking your pulse while you exercise or, better yet, by strapping a heart-rate monitor to your chest or using the heart-rate readout on cardio machines. The faster your heart thumps, the more effort your body is exerting. How do you know what number to aim for? To figure out your "target zone," use a calculator like those at About.com or The Mayo Clinic's website.

Q: How much exercise do I really need to do?

A: The answer depends on whether your goal is better health or weight loss. If you're a couch potato, you can cut your risk of developing coronary heart disease and type 2 diabetes by a whopping 50 percent simply by accumulating 30 minutes of physical activity a day. Boosting your activity to 60 minutes will cut your disease risk by an additional 10 to 15 percent.

If you're trying to slim down or maintain weight loss, 30 minutes of exercise might not be enough. Of course, your eating habits and genetics play a big role, but in general, people who are successful at weight loss tend to be active at least an hour a day. Research conducted by the National Weight Control Registry, which has tracked several thousand people who have lost at least 30 pounds and kept it off for several years, found that "successful losers" reported burning on average 2,800 calories a week through physical activity, primarily walking. That's seven to 11 hours a week, depending on their weight and intensity level.

What if you don't have that kind of time or motivation? Don't throw in the towel! Any exercise is far better than none at all. In fact, a new study published in the Journal of the American Medical Association found that just 72 minutes of exercise a week can improve the fitness levels of postmenopausal women who are sedentary and overweight.

More Q & A:

• Weighing Inventory
• Counter Fitness
• It's Not the Sweat That Counts
• Yoga & Your Weight
• No Playing in the Pool
• Weight Loss at Night

About Our Expert: Suzanne Schlosberg is the author of The Ultimate Workout Log and The Essential Fertility Log and coauthor of Fitness for Dummies and The Fat-Free Truth. She's a health and fitness writer living in Bend, Oregon.