Yes, walking could be thought of as a tool to let the mind wander but I think it does more than that and is so widely available to most that it could be turned into a good habit. It's possible any movement would do but how available is swimming for example?(or running in a lunch break without worrying about the sweaty clothes)... It's also easy enough that one needs not build up motivational energy to go for it, could be just an auto pilot habit that never stops on giving back.