Walking offers the great advantage of requiring no skill or practice. Everyone knows how to do it, and the only equipment you need is a good pair of shoes. You can walk outdoors or indoors (in shopping malls, for example). It is probably the safest option of all, with the least chance of injury.
The main problem with walking as a principal aerobic activity is that you can easily fail to do it strenuously enough to get the conditioning benefits of exercise. Aerobic walking cannot be casual or intermittent, and it takes a little more time than the other options. You should be able to walk about three miles in forty-five minutes. If, after building up to that, you still do not get a good workout, you will have to walk faster, do some uphill walking (long, gradual hills are best), or carry hand weights. (Never use ankle weights, which can stress joints and lead to injury.)
Good posture is important during walking exercise, and swinging the arms opposite to the movement of the legs makes for a better stride. Walking with special poles (like ski poles) is another option. Athletic supply stores can give you advice about good walking shoes. The right shoes are as essential for aerobic walking as for running. If you become accomplished at walking, you can join walking tours throughout the world or look into race walking, an advanced technique taught at some fitness clubs.
Find more information on healthy exercise from Dr. Weil in the Exercise and Fitness section.