Here is a great tutorial series about creating terrains in Unity.
And one tip: using a heightmap generator like HME or Terragen classics saves a lot of time. Both tools are free (Terragen for non-commercial use only) and both can generate and smooth heightmaps automatically. These tools are also great for generating skyboxes.
There are also some packages on the official Unity homepage about terrain:
- Terrain-Toolkit extension (inlcudes a comfortable terrain generator)
- Additional terrain assets
- Unity island demo
- Unity sewer control room demo