Load CK-tools from download sections and import it to the project. Switch to 3D view.
Use "track copy" to copy the original track. Change the copy to active. You can rename the track to trackterrain.. Hard to think correct term.
Change the new trackterrain crossection to "20materials". Delete all but two outermost materials and points.
Switch to "Material" and change the track material to point to your grass texture. Switch back to "Shapes" and drag a lasso to either left or right utmost points. Keep shift pressed and drag those points until they are close to tarmac edge (where the terrain is suppose to start).
Zoom closer both views and finetune the edge. You can add a new point to crossection by crtl-clicking the line in Shapes view. Easy and fast way to "merge "the two surfaces is to create a new point close to the edge and tucking the edge thru the original track face*. Now zoom out and drag the new terrain to required width. It goes 100m from road center without problems but i do suggest you add new material every 20m or just do one "ring" of terrain. You can continue the terrain from the new track/terrain edge to use proper tools after that. Repeat for the other side of track. Add more shape points and add random x/y to the points in crossection to make the terrain not-artificially-flat. try putting extreme figures first to see the effect, then use the more properly scaled figures.. 0.01 to 1 is about the scale needed. 0.10-0.20 is good for normal track.
Add new Material and Shape points to track and adjust when needed. For ex pits and crossroads require you to remove the trackterrain by adding Material change to required start and end of the section and removing the material. It removes the mesh too so no problems ingame.
*Better way would be to use the same original track to create both the tarmac, kerbs and some grass. Do a track copy and try to replace your track with for ex "GP" crossection from CK-Tools XPack... You can use the 20Materials crossection as a "sandbox" to create all sorts of shapes.. Walls, cliffaces...
