One oddity of the rivers is you have the headwaters concentrated in a few small arcs, and then everywhere else is evidently bone dry.

Also, the biome and the geomorphology are two different things (They interact with one another of course). Forests can grow on hills or in plains. Generally what determines whether there's forest is a combination of rainfall and temperature. Where there's enough water and it's not too cold, you get forests, otherwise you get grassland, scrubland, or desert, unless humans interfere and cut down the forests. Keep in mind that most of Europe and eastern North America were covered by forest. It's a good idea to place your pre-agriculture forests, then remove them based on where there are people, and if you are after the temperate climate of typical psuedo medieval fantasy, then you should have a lot of forest, at least to start.