Walking — like swimming, bicycling and running — is an aerobic exercise, 【B1】 builds the capacity for energy output and physical endurance by increasing the supply of oxygen to skin and muscles. Such exercises may be a primary factor in the 【B2】 of heart and circulatory disease. As probably the least strenuous, safest aerobic activity, walking is the 【B3】 acceptable exercise for the largest number of people. Walking 【B4】 comfortable speed improves the efficiency of the cardio respiratory system 【B5】 stimulating the lungs and heart, but at a more gradual rate 【B6】 most other forms of exercise. In one test, a group of men 40 to 57 years of age, 【B7】 at a fast pace for 40 minutes four days a week, showed improvement 【B8】 to men the same age on a 30 minute, three-day-a-week jogging program in the same period. Their resting heart rate and body fat decreased 【B9】 These changes suggest 【B10】 of the important — even vital — benefits walking can 【B11】 about. Walking 【B12】 burns calories. It takes 3,500 calories to gain or 【B13】 one pound. Since a one-hour walk at a moderate pace will 【B14】 up 300 to 360 calories. By walking one hour every other day, you can burn up a-pound-and-a-half monthly, or 18 pounds 【B15】 — providing there is no change in your intake of food. To 【B16】 weight faster, walk an hour every day and burn up 3 pounds a month, or 36 pounds a year. 【B17】 your age, right now is the time to give your physical well being as much thought as you 【B18】 to pensions or insurance. Walking is a vital defense 【B19】 the ravages of degenerative diseases and aging. It is nature’’s 【B20】 of giving you a tune-up.