Try this, colour the water black, and the land 50% grey, then use the Burn/Dodge tool to lighten strips, draw a white ridge with ridges comming out. This will build you a black-white heightscale to use in wilbur, but instead of random clouds you get moutain ridges where you want them. Dont worry about detail, just need to rough it in. After Percipitation, Incise Flow (blur 1, 2 if massive map), Incise Flow (blur 1), Incise Flow (blur 0 is smaller map), then percipitation again twice. (thats my wilbur standard) Wilbur will do alot of work/detail. After i have the wilbured relief, i render it with out rivers (like you did) path some black rivers in the right place (following the contures in your relief), then slap that black river down on your unrendered relief. Once all the rivers are black and on the relief layer, rerender your new relief layer. This will cut channels for the rivers.