One of the biggest mistakes people make when they are walking for exercise is that they don't exert themselves enough, says Dr. James Rippe. To receive maximum cardiovascular benefit from walking, you must get your heart beating at a target training rate.

To determine the range of your target training rate, subtract your age from 220 and multiply that number by 0.8 (80 percent) and by 0.6 (60 percent).

For example, if you are 40 years old, you would subtract 40 from 220, leaving 180. Then multiply 180 by 0.8, giving 144, and by 0.6, giving 108. The higher number, 144, would be the maximum number of heart beats per minute for your exercise goal; the lower number, 108, would be the minimum.

Walk for at least 10 minutes, then measure your pulse for 30 seconds, either at your wrist, behind your ear or along the side of your neck. Multiply the number of beats by two. This number should fall between your minimum and maximum training rates.

Robert Sweetgall, who has walked more than 22,000 miles in 21 months, also offers this walking advice:

To start a walking program, begin with a mile or two almost every day, some days easy walking, some days hard walking. "If a person walks one mile everyday, over and above the current level of activity, they would drop seven pounds a year if they never changed their diet," Sweetgall said.

*Comfort is the most important factor in choosing a walking shoe. Make sure your feet have plenty of room, and be sure that the sole is stiff.

*Wear absorbent socks, preferably cotton or wool.

*Keep two pairs of walking shoes, so one can air out while you wear the other.