I just wonder, what's the most practical way of doing it? I've tried extruding parts of the road inwards, but in curves I get problems and there's much manual work involved to fix it, so I'm afraid to end up using lots and lots of time, and get a bad result. In fact, I believe it would end up as an unholy mesh.

The other way of doing it, which I'm pondering now, is to use a separate plane object and then model it to follow the road, with no visible gaps between the road and the terrain mesh. This, in my view, seems like a better way of doing it, but of course I'll have to use much time for it to look right, with no visible gaps and all.
Are there any better options I've overlooked? If not, what would you choose in my situation?