I’ve had a tip that you can ride from the top of the GT black route over the hills to Innerleithen which I was considering doing on Sunday.
You can, but I wouldn’t as it is crap. What you can do is ride Inners and then take the golf-course road following it for a few miles before turning left for Leithen Water and then climbing to the Boundary Trail and the continuing on Black. Using the link path to get back to Inners.
Take a look at the other Tweedlove stuff, some is free.
Monday – Natural Tweed
Tuesday evening – Selkirk locals
Wednesday evening – Alpine bikes ride plus BBQ
Best thing would be to explore Strava along with an 1:25000 os map, loads of trails on/off the Three Brethren/Minch Moor ridge.