Any advice on how you go about it? Building are easier, just look at their function, but trying to get natural irregularity is hard.

I'd recommend having a look at http://www.darkcreations.org/hoddminir/category/tutorials on the subject of heightmaps and generating a worldspace based on heightmaps. GeoControl 2 is used in the tutorials, but there are also alternatives like World Machine. If you are working on a very big worldspace and don't have Photoshop, then you can use ImageJ for simple manipulation of 16-bit heightmaps without having to convert to 8-bit (as far as I know GIMP can't write 16-bit files yet). World Machine has a trial that has a limited resolution on output (max 513x512 pixel), but the heightmaps can be stitched together and made smoother in CK.

Can't you use the World function in the CK? I'm just making a view from a tower that you can't leave. It's small just a 5 X 5 grid, a view of a field surrounded by low hills so it doesn't need to show far away things.