I have a little different slant on this topic - so here goes: I walk because I have degenerative arthritis and need to build muscle mass around my damaged joints. Impact, as in walking fast or jogging, is absolutely out as an option. So, as I started losing weight, I added distance. This really works for me. When I first started walking, I could not make it around the block without stopping to rest. Now I walk 3 miles daily either indoors when the weather is bad, or on a route I mapped out by car to check the miles.
So it really depends on why you are walking as to the right answer for you. I won't ever get into a true cardio mode because my joints can't take the strain. But I can do the distance.
Hope this helped some...Carleen...