Personally I would look into the ADVANCED TERRAIN commands for making terrains instead of the default ones.
Anyway, a heightmap defines the hills and valleys of your terrain. Black means no height, and white means the highest height. Shades of gray are in-between. Heightmaps are very easy to create. You could make them in a terrain editor, a program like GIMP or Photoshop, or even in Microsoft Paint. All you have to do is draw your hills from a top-down view in black and white and save it as an image.
If you're using Advanced Terrain, there is a very easy command you can use for walking on terrain. It is called GET TERRAIN GROUND HEIGHT and works in the same way as the GET GROUND HEIGHT command that you would use on a matrix.
There is yet another terrain system out there called Blitz Terrain that controls Level Of Detail (LOD) for you. I've heard it's pretty good and fast.
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