Here is a selection of excellent pre created, pre textured terrains to use as a start for your game. Complete with grass and tree's already placed.
Simply replace the splat textures, tree prefabs and grass textures with your own. Resculpt, repaint to your needs and away you go.
All textures and models are from Unity's packages.
More will be added on a regular basis
There are currently 15 variations of 8km islands in this pack.