Your world after 200 hours will be awesome, do not reroll a new seed. The world map is massive and even if you have uncovered an island or two with Mistland biome present it will not effect the massive Mistland areas you will eventually explore. It is beyond me why people contribute so much negative misinformation on forums. The designation of Troll was wisely chosen, as they are mean, malicious and butt-ugly.

We had uncovered probably close to one half of our map, but the majority of that was around the equator. We had even discovered and explored a couple of Mistland biomes. Believe me when I say we did not even scratch the surface, and those Mistland biomes we had uncovered, still had virtually all of the Mistland creatures populating the zone. The only thing missing was Dvergr buildings & Infested Mines. One of the continents we had partially uncovered some Mistland biome, which still had a lot of Mistlands unexplored, was packed with creatures and had spawned an archipelago of Mistland islands full of Dvergr buildings and infested mines.

Unless after 200 hours of playing your seed you intentionally uncovered vast swaths of Mistland biome, I would not reroll a new seed and abandon all the work up to this point.

cheers.