Once you have a heightmap I'd eyeball where you want the road to go (including switchbacks) and then using a road width brush, smudge/convolve stroke the path. This will average out the height and make the road even. You may have to reverse the path direction on alternate strokes.

This will put the flat road into the heightfield. Maybe. I haven't actually tried this technique for this purpose, so YMMV.

-Rob A>