Sorry for the double posting, but I didn't resist making an actual experiment. I (Elena) walked from the Port Authority to the Miller's Daughter (from door to door, without getting in or out) in 4:15 minutes. This corresponds, approximately, to 2 hours and 8 minutes in game. It is not exactly a straight line; there are some turns and there is also the time necessary for the zone transitions. Still, it would be more realistic, I think, to say that a PC can walk 3 zones in two hours than in one.