I don't know. Maybe the fog concept could work, but I'm not sure people would like it because it hides the gorgeous views Larian has created. Plus, it might tax low end computers and thus raise the minimum requirements. But maybe.
I frankly think it's just be better to cut the dang surface map in pieces, creating separate areas. Smaller maps with gates to others would instantly create the impression that PCs traveled for hours between locations. You wouldn't even have to say, "Three hours later." Just the fact that once you leave the beach/nautiloid/crypt map, the game transitions to the grove map would imply the passage of time. Same with going from the grove area to Moonhaven/The Forest. Same with going from there to the Bog.
Yes, it would be considerable rework, but any revamp of the map at this point would be considerable rework.
They probably won't, so we'll probably just have to imagine it in our minds. As you pass into the Forest, close your eyes and imagine you've spent hours getting there. Now open your eyes. Ah. There you go. Area transition.
