翻译划横线的句子：
(46)I first took up walking as a means of escape. (47)After a busy morning in my office, I found it refreshing to take a stroll at lunchtime, to breathe the fresh air and feel the sun. Another walk in the cold night air was, I discovered, an exhilarating way to unwind. (48)Walking--like swimming, bicycling and running--is an aerobic exercise which builds the capacity for energy out-put and physical endurance by increasing the supply of oxygen to skin and muscles. (49)Such ex-ercise may be a primary factor in the prevention of heart and circulatory disease. (50)As probably the least strenuous, safest aerobic activity, walking is the most acceptable exercise for the greatest number of people. Walking at comfortable speed improves the efficiency of the cardio-respiratory system by stimulating the lungs and heart, but at a more gradual rate than most other forms of ex-ercise.