Hi grimleyblack I know I've been waiting to see where you go with this, but I can make a few comments now, if that would help you

1. Your rivers are good. They run from high ground to the sea, they join properly, and meander a little bit.
2. Your mountains are very dark, a bit too dense to see well, but they do run in proper linear ranges. The one mountain junction at the center of the map is a little to much angle to be natural -- mountain ranges normally don't join at right angles.
3. The blue shallows are too bright, and the lighter color spreads onto the land. I'd make the shoreline more like the river edges, a bit darker than the land.
4. The shorelines are just a little too smooth for my taste. You could make more small bays and peninsulas to break up the smooth look.
5. The color difference between your hills and your mountains is too great. It makes it look like the mountains are just sitting on top of the plains, rather than an inherent part of the land. You might try color changes, and connecting them a little better, so it looks like the hills are just the smaller eroded mountain edges.

That said, this is a good start, let's see where you can take it

Cheers,
Cornelia