"Walking is man's best medicine." Hippocrates (The founder of Western medicine)
Walking is one of the most simple - and cheap - forms of exercise. Almost everyone can do it, and it’s a great way to get fit and healthy; lots of the research out there on exercise and chronic disease prevention uses walking to test the impacts of exercise.
Walking also has the benefit of being a low-injury exercise (assuming you don’t do it in front of a bus!), and compared with running the lower impact-stress of walking means it’s easier on joints like knees, hips, and ankles.