There are tons of apps out there, I use Endomondo, but Strava I think has become the most popular. I don't have iphone but I think most of them are multi platform for all the common mobile devices but not sure. I think maybe Map My Run is another popular one. Realize these are mainly for running which is out in the open. If your just walking (I don't recall that you were a big time runner) it will work just fine if out in the open but if your walking through buildings and stuff like that GPS signal will be lost then you might need one of those step counter thingies instead.
