Well if your using Blender, use the edge select tool and select all the edges along the side of your path, then extrude it. That will create your wall. You then select all the faces of your wall and press P in edit mode. That will separate your wall from your terrain mesh. If you dont apply a material or texture to the wall it should be invisible in panda. If your using Bullet for physics I think they have a “ghost” mode that will set invisible objects so that you cant pass threw then.
Another option is to create a duplicate of your terrains path and set it so that anything in collision with the path moves normally, other wise all objects move at a much decreased speed. So when your not on the path your character moves as though it is walking threw wet cement.