Not ridden it but planned a (mainly) legal circuit round the Cheviot from Wooler side. Get an OS map and its obvious.
Also done some bits round Belford area on paths and bridleways from Belford out to St Cuthberts Cave then round Kyloe Woods and back over to Belper. All very easy, i.e. not technical, but reasonable fun and not too boggy.
Good luck and let us know if you find anything better.