well it takes burning 3500 calories to lose a pound i believe
caloriesperhour.com has a calculator for running, it says that if you're 120 pounds it would take running (not sure what speed that is considered) about 45 miles to burn about 3500 calories.
try going here- http://www.caloriesperhour.com/index_burn.php
find walking, or running, they have different speeds, and you enter your weight, and time it took (just start entering some estimated times until it tells you that that burned 3500 calories) and that should give you a more general guess.
but you burn calories just by being alive, so it's not like you'd have to run 45 miles to lose a pound, that's just if that was the ONLY thing you were doing, when you burn calories by doing nothing anyways.
hope that helped, or hope i at least didn't sound like i had no clue what i was talking about :P