I also know what you're talking about in regards to the radius of the locations and the distance between them all. The range between some of the locales seems to differ at times. Remember in the episode with Morgan when Carl was looking at a hand-drawn map of the town on the wall and pointed out their old house and the police station? I wonder if that gives much insight into the overall location. I may go rewatch that episode just for that part.
Where Morgan lives, Rick's Hometown, is King's County, Georgia. It's not a real town, but I would imagine it's quite a distance from the prison, since it took the whole morning to get there, I imagine they left early in the AM that day. As for the distance from Woodbury to the Prison, in the book it was approx. ~10-20miles. During that episode with Andrea, she left early midday and didn't get to the prison til early the next morning...sounds about right to me, considering all the side tracking she did.
https://maps.google.com/maps/ms?ie=UTF8&oe=UTF8&msa=0&msid=211974457044924343699.0004723d3ecc55ac63583 that even Kirkman recommends, it's from a fan and based on the comics, but the only real difference is Rick is from Kentucky in the comic, but Georgia on the show.
Now, one big thing on the show is a sense of time, we clearly don't know it on a lot of episodes. In a few episodes this Season, some episodes spans the course of two days while some only span the course of an hour, we just don't know unles it is clearly stated by them on the show.