When trying to use Unity as it was ment to be used: Creating a terrain and texturing it.

The terrain texture

Guess I need a texture, but since I’m lazy:
1: Open Photoshop
2: 2048×2048
3: Filters->Render->Clouds (remember to adjust colors first)
4: Filters->Pixelate->Crystallize
5: Done

Apply adjustment layers to change colors; It’s the best way.

Making it seamless

You could either check other guides by googling, or just follow this summary of 75% of all guides:

1: Filters->Other->offset (select 50% canvas size in both directions)
2: Go crazy with the spot healing brush

The normal map

In case you didn’t know, a normal map adds lightbased depth to your texture, making it seem a lot more realistic. Check Unity’s own guide.

With your texture still in Photoshop, just create a normal map by using photoshops Filters->3d->Normal map and fiddle with the knobs until it looks interesting.

The heightmap

A unity terrain is hard to mold with Unity in-editor tools, so rather create a grayscale heightmap in photoshop. Generate a blank canvas at 512×512 grayscale. (other sizes ok)
Then use Filters->Render->Clouds.

Save as photoshop raw and import to unity. (google it)
Important: When importing, you get a little popup that asks for x y and z things. Make sure the Y thing is not set too high because it’s meters from topmost point to lowest point on your heightmap.

Use a large brush to fix your heightmap.

Some results plz

Here are some seamless textures along with a normal map that fits them all.

The resulting landscape is in the top. Closeup here: