Here is a rough map of the better of the two routes.
When you are at Innerleithen XC just follow the XC route until you get to the top of Minch Moor then turn right at the first split in the descent.
If you let me know when you are heading over I may be able to show you round the place if I'm not busy.
Route length is roughly 35 miles but it is far from being flat – at a guess 7000-8000ft of ascent. Descents are fantastic though.