In reply to Tim Chappell:
PS What I have done is White Bridge to Glen Tilt. There's a lot of pushing in that trip. It's not an unpleasant push, far from it, but it's not really rideable between the old house a couple of miles S of White Bridge, and the beautiful watersmeet on the Tilt about a mile above the Bedford Memorial.
Not an unpleasant push, that is, assuming an unloaded MTB. I'd *hate* to do that stretch with panniers or, still worse, a trailer.