I have he same thing as you OP. Still using the map I set up at my start a month or so after game release... still not explored it all (OK I may be slow but why rush?) You'll find some have no doubt!

The "Upgrade World" mode has been my go to after every release of content and it does it exactly what it says on the tin. In fact considering using it to reset & refresh my world (bar the buildings) and inviting friends over ;)

Thanks very much for the help. I will start sailing and see what comes up & share.

If you go to unexplored region of mistlands, it should be okay. It may be slightly different from the intended experience, but on my own part I can say that even older world seeds should have approximately same amount of features as new worlds.

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.