A recent post on the Swords of Minaria about how sparsely populated the lands in a default LBB D&D campaign seem to be makes me want to re-visit the topic of post-apocalyptic D&D. I don't think the standard early settings were post-apocalyptic, as I've said. Just because there are remnants of a long-dead empire as part of a setting doesn't make the setting post-apocalyptic; there's got to be a statute of limitations. But that doesn't mean that you can't think post-apoc to make a more interesting setting... and large groups of wandering humans in an otherwise unsettled wilderness certainly suggests post-apocalyptic.
What might be a good move, especially when trying to build a game world as you play, is to limit the known world to no more than three smallish kingdoms. You really only need one: the home base. You might want one or two others to establish political tensions and provide options for play. These small kingdoms are surrounded by vast, mostly unexplored wilderness, simultaneously frightening and tempting. The kingdoms want to expand, which is why moving into a wilderness area, clearing the hex, and establishing a stronghold is encouraged.
There may be other kingdoms in the world, but they are very distant, out of touch, and considered legends. There's very little known about these mythical kingdoms. When contact is finally made with one, the kingdom should be weird; it's not like the known world, and the GM can feel free to make up random customs and factoids about the new kingdom without regard for what the players have come to expect as "normal". Establishing trade routes or political alliances with such kingdoms can be an adventure in itself, especially since there will be wild seas and untamed wilderness isolating the new kingdom from the known world.