At the end of last season, didn't the camera pull way up from Rick's group, then pan over a bit to reveal the prison? If so, how is it that 6 months later they're only now finding this relatively nearby location?
Because they've been walking in a circle, around it all this time. When they pull out the map on the car, and start talking about where to go, they state that they've been going in circles, and back tracking. The herds are moving, and they can't just go in a straight line. Plus, they often tend to stick to roads, and this prison looked like it was out in the middle of no where. They just finally happened to come up on it this time.
What I wonder though, is why it didn't show on the map? If it were me, and I were looking at places to go, I'd study the map and look for secure facilities listed on it. Not sure if they put prisons on maps though.