One easy way to edit terrain is through the WorldPainter app. It is available on Mac and PC and is fantastic for making amazing terrains in a snap. It only takes a few minutes to grasp the tools and after about an hour of experimenting, you'll be able to make massive maps in no time. For example, you can probably make a 1000x1000 mountain range in 10 minutes. This app includes:
-Terrain editing
-Tree generation
-Resource generation
And much more...
The only downside to this mod is that is it what is called 2.5-D. This just means that you can only build in a 2-D manner, but have control over heights and contours of the map. For the third dimension, export your created worlds and reopen them in MCEdit. It may take a little longer to grasp THIS program, but it is still great for making overhangs and arches as well as copying and pasting previously made sections of a map(With the export tool, you can select one region, let's say a house, and create a schematic. From there you can import the schematic, rotate, flip, mirror, roll and scale that schematic in a different map.).
I wish you luck.
