Originally posted by Edgar:

keep trying, and move swift. the entire outside of the map is mistlands. very doubtful youve completed more than 5-10% of it.

It's actually not that much. Far North and Ashlands wrap around the western and eastern edges almost a third of the way up (it might depend on world seed, but i suspect not).

If your playing on a long term world (which solo players usually are) I'd recommend only exploring one side, east or west, incase mistlands gets new content added like the other biomes did.