Although it may seem like a soft option, in fact walking is one of the very best forms of aerobic exercise. A 6-mile walk burns up only 20% fewer calories than a hard run over the same distance. An excellent choice for anyone who feels low in energy or is convalescing, walking is also particularly convenient because you can incorporate it into your daily routine to get you from A to B.
To begin with, decide to walk for 30 minutes a day at a pace where your breathing becomes heavy but not strained. Make sure you wear comfortable clothes and good walking shoes. If you walk to work you can carry another pair of shoes with you to change into.
After a week of walking for 30 minutes a day, go a step further. Try walking for an hour and see how far you get. As a guideline you should be able to manage 3 miles in an hour at a leisurely pace. As you become stronger you might like to try hill-walking or even running.