Nice little landmass so far. I like the subdued colours and the overall feel of the piece.
So who when you say that the squares are 50 mi.˛. Do you mean each square is seven points something miles by seven points something miles? Or do you mean that each square is 50 miles x 50 miles? (Actually 2500 mi.˛) Looking at the map I think you mean the latter, because that would make your landmass about 850 miles across. This would seem more appropriate.
There would be huge amount of terrain variance in a 50 x 50 mile square. So anything you draw in their is only going to be a general feeling for what is in the space rather than getting to specific and detailed.
for instance in the city I live there it is an urban sprawl about 70 miles x 50 miles. Within that area there are lakes, suburbs, towns, forests, a bay, mountains, farmland, hills, about 100 rivers and creeks and so on.