How do I fill the space in the centre of my circuit in 2.79? I'm learning how to make custom tracks for a game called BallisticNG and it requires me to use version 2.79 of Blender specifically due to a neccesary addon not being updated for 2.8. 



In the above image is the basic track I created within the game's track creator and exported as an OBJ for the purpose of adding terrain in Blender. My current goal is trying to fill the center hole along the top most edges of the inner wall. I've tried the Grid Fill option but for somereason it just doesn't want to work with this track.

File Here:
If I understand correctly, what you could do:


First clean your mesh a bit, make W > Remove Doubles to remove the doubles and merge vertices, and untriangulate with AltJ.





Now as you have merged the vertices you can select the rim with a simple Alt left click, fill the face with F and join some vertices so that its more blender friendly. It is still a ngon though, do you need a quad grid?