Optimal way to fill terrain between roads

TL;DR: Need to fill inside and around the roads with mountains and landscape, using a single mesh, don’t know how to do that most efficiently.

I’ve made a road around an A.N.T. Landscape generated mountain with array and curve modifiers, applied them, got the road.
Now it looks like this:

Is there a way to add inside and outside of the roads some landscape mesh without having to do everything manually?
Every way I try, there is something that seems to be an unsolvable problem:

• I could cut out the place for the road from the landscape with the Knife Project tool. problem: I don’t know how to actually merge vertices from the road and the terrain next to it, without having to merge them all one by one.
• I could fill the in-between parts with faces somehow (which I’ve done in a rather disgusting fashion):
  
  I could shrinkwrap the inside part on the landscape mesh, but problem 1: I haven’t found a way to apply shrinkwrap modifier to only a certain part of a mesh, and I do not want to shrinkwrap the road.

I could just create a landscape out of the green parts with sculpting but problem 2: I don’t know how to apply sculpting only to a certain part of the mesh, so sculpting will deform the roads as well, which I can’t have.

In the end, it doesn’t matter how it gets done, I just need roads and landscapes in the same mesh, with both having appropriate topology. If there is any other technique and I’m way off, please let me know.
I need to achieve something like this: