DEAR DR. DONOHUE: I’ve been walking five miles on a flat trail. It takes me 35 minutes to walk 2.5 miles, and then 2.5 miles back. I started at the end of October. Recently, I have had to stop because my shins hurt real bad. Would warm-ups help? Or are my shoes the problem? I am 51, 5 feet 10 inches tall and weigh 220 pounds. The trail is a dirt trail. I wear regular tennis shoes. — T.C.

ANSWER: Let’s take care of your shins first. The shin is the tibia bone, the larger of the two lower leg bones. Shin pain often is dismissed as shin splints, a diagnosis that has no meaning. A fairly large number of conditions cause shin pain. One is stress fractures of the bone. They’re tiny cracks in the bone that come from overuse. (You’re not talking about calf pain, are you? That’s a different problem that has to do with circulation.)

Take a break from walking until the pain has completely gone. If it hasn’t gone in four weeks, see a doctor. If it does go, then you can start walking again, but reduce the time and the distance. You can gradually pick up your pace and extend your distance so long as you are free of pain.

A warm-up for walking is simple and a good idea. Walk at a very relaxed pace for five minutes. Stretching the leg muscles would be helpful, too.

Shoes are important for preventing shin and foot pain. Buy some walking shoes. The most expensive are not always the best, and neither are the most padded. When you go to buy the shoes, wear the same kind of socks you wear while walking. You ought to plan on buying a new pair every six months.

I’m not clear about your walking speed. Do you cover the entire five miles in 35 minutes (a fast running pace), or do you walk each 2.5 miles in 35 minutes. If the second is true, you’re still walking at a good pace and burning about 10 calories a minute. Maybe your walking posture is contributing to the leg pain. Walk with eyes focused forward and with head above the shoulders, and shoulders above the hips. Your arms should be bent, and the right arm swings forward as the left foot hits the ground. Strike the ground with the heel of the foot and take off for the next step from the toes. There is much discussion about proper foot strike, I admit.

DEAR DR. DONOHUE: My friend and I, both in our late 60s, have joined a yoga class. We love it. Neither of us has done any formal exercise until now. What do you think of yoga as exercise? — H.S.

ANSWER: I think yoga is excellent exercise. It increases flexibility and keeps joints limber. Unless your class moves rapidly from one position to the next, it’s not the best heart exercise. You should add some aerobic exercise to your program. Brisk walking is a good choice.

DEAR DR. DONOHUE: Can exercise cause diarrhea? I believe that’s what has happened to me. I jog every day. The past month I increased my mileage and speed. Ever since, I have to hurry home to get to the bathroom quickly. The only thing I am doing differently is my jogging routine. I haven’t changed anything I eat. — P.K.

ANSWER: Walking, running, biking and swimming all can cause diarrhea. Some believe it’s due to the jostling the stomach and digestive tract take while exercising.

Another explanation is that the exercising muscles divert blood from the intestine in order to supply it to the exercising muscles.

A third possibility is dehydration.

If you’re jogging after a high-fat or high-fiber meal, that might be the problem. Such meals stay in the stomach longer than does a high-carbohydrate meal.

I believe you can overcome this problem by going back to your former schedule. When things have quieted down, then you can increase your mileage and speed gradually, never more than 2 percent a week.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.