What are the altitude ranges for each range? You're telling Wilbur to fill the selected area with fractal noise from 0 to 1 (Amplitude), which will show as constant green If the maximum value in your map is much above 100. As Greason Wolfe suggests, use multiply as the operation if you want to keep the maximum altitude roughly what's shown in your map. If your ocean is 0, using multiply can let you apply the fractal noise to the entire surface at once without having to select each altitude individually.

I don't recommend using a surface that large, especially for initial roughing in of the terrain. Fill Basins is likely to fail with a crash and performance using Incise Flow is likely to be "maybe it will finish". Starting with a much smaller surface and scaling up to your final size is much more likely to be effective.

The shown portion is a crop of a much larger piece, correct?