It depends upon how close you explored to the Mistlands when you last logged in. I'm about where you were, perhaps 20% and only in a single exploration path, as my Boss stones were somewhat aligned....and I've found plenty of Mistland content, not on the areas I exposed earlier, only get the mobs there, not structures, mines, POIs[AND NO MIST!!]...it wasn't that much, only a few small biomes and a good sized one on an island. It really depends how much fog uncovered in a Mistland biome area.

good luck!

PS: using my original map seed 1200 hrs/GameDay 2017...for what it's worth.