One thing that works is to get some 1'x1' plywood squares and 2" foam.
Embed nails and/or screws through the plywood to reinforce the foam and make sure they allow you to shape the terrain the way you want. Use glue and mount the foam to the boards, then measure things out for consistency (The friend of mine I helped with a few uses 2" intervals) with a marker and halfway down the foam just to make things easier.
Work it into the shapes you want for the pieces, just keep things consistent for modularity such as which sections rivers or roads meet with.
I'd suggest that you still keep hills separate until you have the rest to at least your basic level, but for them, just use larger reinforcement options and more foam.
After you shape it, take some filler to work it a bit better for the look and making it a bit more fitting, and lets you do things like cobble or similar for roads. On a solid surface, you can actually use hot glue dragged across it for dirt tracks. Scattered rocks and other things, perhaps stones for a streambed.
Then finish texturing and paint, flock, water, and whatever else.
Just make sure you write down your paints, flocks, and everything else to keep things matching.