DEAR DR. DONOHUE: I have some questions on walking as an exercise. Does it really count as exercise? How much do you have to do? How long do you have to walk? What’s the meaning of 10,000 steps? Do you have to do the entire walk at one time, or can you break it down? Are there any rules for the proper walking form? — E.R.

ANSWER: Walking is recommended as an exercise because just about everyone can do it and it requires no special equipment and no gym fees. It is a true exercise. Taking even a few, slow steps is infinitely better than sitting.

The numbers I give are approximations. This isn’t an exact science. Brisk walking is good heart exercise for the able-bodied. “Brisk” is defined as walking at a pace of 3.5 miles in an hour, the same as walking one mile in 17 minutes. If that is too fast for you, don’t worry. As you become used to walking, you can pick up the speed. Others define the best walking pace as taking 100 steps a minute.

The ideal duration is 30 minutes. Start with much less time.

You can break your walking exercise into three sessions of shorter duration.

Walking form? Purists tell us to walk with head erect, eyes directed down the path, arms bent at the elbow and back straight. When the left leg moves forward, the right arm should come forward, and the same thing for right leg and left arm. Land on your heels, and step off with your toes.

The 10,000-step program is the equivalent of walking five miles. You’ll go crazy if you try to keep track of all the steps you take in one day. Don’t try. A pedometer, a little gadget that keeps count of steps, is available in all sporting-goods stores and is relatively inexpensive.

DEAR DR. DONOHUE: What’s considered the best sit-up? I want to take some inches off my waist. — D.L.

ANSWER: No exercise selectively removes fat from the body area being exercised. All exercises burn calories. A calorie deficit makes the body turn to fat for its fuel. That fat comes from all the body fat-storage places.

Sit-ups build stronger abdominal muscles, which will act like a girdle to hold the abdomen in.

The “perfect” sit-up doesn’t exist. An “imperfect” sit-up is one done with the legs stretched straight out in front of you. That exercise is hard on the hip, groin and back muscles. Keep the knees bent to 90 degrees and the soles of the feet flat on the floor. Raise your head and shoulders off the floor while simultaneously squeezing the abdominal muscles. You only need to raise the head and shoulders about 30 degrees. Hold that position for a few seconds and then slowly lower yourself down to the floor.

DEAR DR. DONOHUE: Three friends and I, all in our 50s, have joined an exercise class in step aerobics. We love it. In three weeks, I have lost five pounds.

I have noticed a funny feeling in my knees. They aren’t sore, but they feel like they could become sore. What should I do? — R.O.

ANSWER: For other readers, step aerobics is an exercise that employs a small platform that’s the height of a stair step. The exerciser steps up on the platform and then down from it. It gives a good workout.

Maybe you’re landing incorrectly. The entire foot should land on the platform. The heels should not be sticking off the back. The entire foot, therefore, absorbs the impact of landing, and that spares the knees from getting a too vigorous jolt.

And also pay attention to the bend in your knee. Don’t land with a fully bent knee. The angle of the knee bend should be less than 90 degrees.

Frankly, I’m not positive what a “funny” feeling means.

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.