How do I get good rivers in Wilbur?

[Kyrel]

I have the above height map created in Photoshop (essentially clouds with some inverted Difference Clouds in patches to act as mountains). Now, I want to have as close as possible to 'realistic' rivers, but attempting to create these in Wilbur is a bit of a pain; I tried to follow the tutorial here Fun with Wilbur, Volume 5. First off, using 'fill basins' gets rid of all my lakes in the north; they just become flat land, which is not what I want. Then, I tried Texture > River Flow on the raw map, and I ended up with blue everywhere, essentially. It is a big map, 5k by 5k pixels in size, but does that mean I really can't get 'average looking' rivers? I've tried doing Incise Flow as well, as indicated in Fun With Wilbur vol 1, but that doesn't seem to do anything with the settings given in that tutorial.

I really don't want to have to come up with rivers 'manually', because that feels like cheating and it's not as realistic... so knowing how to properly 'riverify' this map would be great...