Here's a single terrain image that squeaked out during my lunch break. If you want to work within the Terrain Editor instead of modifying a bit map image, it works best to combine two or three Generators and then apply the filters of your choice. Reordering the filters or generators can make significant changes to the landscape.

Don't like what you see? Press the Shuffle button in the lower left corner of the terrain editor to create a different terrain using the same parameters. This is also quick way, after duplicating an existing terrain, to create a new terrain that looks like it is still part of the same geology. 

Just food for thought.