This confuses me as well. There really should not be enough zombies in rural Georgia to fill upp that fast. They even seem to kill them on a daily basis. Perhaps they should increase their efforts. They do not seem that short on time, either.
Did you see the teaser for next week with like 7500 walkers? They come from urban Georgia. As food in metro areas is depleted, they would begin moving to outer regions, along major roadways, because they're more easily traversable. Plus, that little tribal world war between Woodbury & the prison 6 months earlier likely alerted any walker in a 20 mile radius. All suggestions indicate that the prison is near Senoia GA, just 20 or 30 miles down Rte 85 from Atlanta, right in a triangle of Macon, Columbus & Atlanta. If you started slugging along in a herd of thousands it could take that long maybe, following sounds like cars, helicopters, gun fire & anything else these people have had going on there in the last 3 seasons. You have to admit, that area around Woodbury & the prison has likely been more active with noise than just about anywhere else in the whole region of that triangle