The huge bonus of walking is that while it is a great cardiovascular activity, it is very low impact. This means that walkers suffer from fewer impact injuries than other exercisers. Numb foot is usually caused by ill-fitting shoes. They may seem quite trivial in comparison to grander joint or muscle injuries, but don't underestimate the debilitating properties of blisters. Not only can they affect the normal stride as your body overcompensates for the pain caused when your shoe rubs up against them, but really bad blisters can get infected.
Blisters result from undesirable friction.
This friction can be caused by your foot sliding around in a shoe that's too big, your shoe pinching on your foot or your socks bunching up under or sliding around on your foot. The best prevention against blisters is to wear proper fitting shoes. The second blister buffer is to wear well-fitting socks, especially ones designed for walking. Another prevention is to slather your feet with petroleum jelly, then put on your socks and shoes. This may seem and feel disgusting at first, but it does reduce the friction that causes blisters in the first place.