Sorry, you are not missing anything, RGE tells the sim where in the world you are, Demex provides the terrain (hills).
You now have to put markers down where the track is going to go. You will need maps for this. Choose points along your track, probably good to put them every 1/4 mile or every 1/2 kilometre so that you can see from marker to marker in RE and hopefully lay your track by eye.
From your map get the coordinates of the points and convert to longitude and latitude (decimal values) and put markers at these points.
If you want to get the gradients correct you will need a gradient profile of the route, (if possible with spot heights) so that you can align the track as near as possible to the correct height and gradient.
(If you are using Demex and MSTS coordinates you can't get perfect gradients and distances because the little squares are not square!)
Have fun

and don't forget to make back up copies of your route all the time :~