I believe there's a mod called World Updater that updated worlds with tar pits/etc with H&H successfully. I'm hedging my bets it should be able to fix Mistlands.

Even then, there's no way I'll be able to explore the ENTIRE map. Should be able to find plenty of Mistlands post update.

when is this update suppose to come?

Yeah just don't come within the radius that would allow objects/trees/etc to spawn, to the Mistlands, actually it might be more lenient than that, I remember still getting a tar pit in a Plains I mostly explored but had one part not mapped and went there after update and sure enough a tar pit. But yeah to be safe just try not to go out too far of the world map.