Q. For exercise, I walk seven miles in 1 1/2 hours after doing an hour of warm-up calisthenics. Which of these exercises is more beneficial to the cardiovascular system? How do I know if I'm overdoing it? I don't feel exhausted from my efforts.

A. If you don't feel overly tired from exercise, you're probably not overdoing it. As a rule of thumb, if you're not so breathless that you can carry on a conversation while exercising, you're working out at a pace that's O.K. for you.

You can also use your heart rate to gauge the cardiovascular benefits of exercise and to make sure you're not pushing yourself too hard. Your maximal heart rate is roughly 220 beats per minute minus your age. Your exercise heart rate -- the goal to shoot for if you want to really condition your heart -- is between 70 and 85 percent of your maximal heart rate. There are other, more complicated formulas to figure your target rate, but this simple one works for most people.

For cardiovascular conditioning, you should exercise to your target heart rate for at least 20 minutes, about three to four times per week. Working out less than this doesn't challenge your heart enough to significantly improve your long term cardiovascular endurance.

Researchers have measured the relative benefits of different forms of exercise. At rest, you burn up about 80 calories an hour. Walking at 3 1/2 mph uses up about 310 calories per hour. Light calisthenics uses about 270 calories an hour, riding a bicycle 500 calories, swimming 670 and running 1,100.

One way to compare exercises is to check your heart rate after each form of exercise; the one that gets your pulse up higher is the one giving you more cardiovascular conditioning.

Q. I am nearly 55 and seem to be having a particularly hard change of life. Can you please tell me what causes hot flashes, which I've been getting for more than two years now. I cannot begin to describe the annoyance of the sweating, soaked bedsheets, interrupted sleep, and so forth. Most of my friends say the hot flashes last two years, at most.

A. The hot flashes of menopause generally decline within three to four years. If they're particularly troubling, I recommend discussing treatment with your physician.

Doctors don't know the exact cause of hot flashes, though they seem to be triggered by a burst of hormones from your pituitary gland (particularly the hormone LH, best known for its stimulating effects on the uterus and ovaries). Falling levels of the female hormone estrogen, made largely in the ovaries, set this chain reaction off. This response, in turn, dilates your blood vessels, ending in the familiar flushing, warmth and sweating of hot flashes.

About 75 percent of women experience hot flashes during menopause, and at least one fourth are uncomfortable enough to seek treatment for them. Estrogens hormones are very effective in relieving these distressing symptoms. Besides coming in pill form, estrogens are now available in patches that you wear on the skin.

If you're unable to take estrogens, another female hormone, progesterone (Provera) may work just as well. A new treatment for hot flashes is the blood pressure medicine clonidine (Catapres), which is available as tablets or skin patches. Though not a hormone in itself, it seems to prevent the dilation of blood vessels and flushing sensation. However, at least one study showed that clonidine was no better than a placebo in relieving this reaction.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington. Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.