I don't actually walk, since a treadmill takes up too much space, but I do shallow side squats and similar exercises while working. I'm a bit hyperactive and found this helps me stay focused better. What you need to do to type easily while doing this is have your desk so that it supports your forearms solidly so your body's movement (and there always is some movement of your shoulders and upper arms when you move your legs) is not transmitted to your hands. I am not an especially fast typist though and someone who is may have a bit more trouble, or may be slowed somewhat.