SUW looks worth an explore crossing the road just after Carsphairn, have you ridden it from Dalry to the Sanquhar end at all Soma?
Back in 94 i rode (pushed/carried?) the entire route with a mate over 10 days………don’t do it, there is far better long distance trails in scotland to explore, There was recent plans to upgrade the very poor sections so that they are passable by bike but i’m unsure as to whether or not they have secured sufficient funding to do this.
Perhaps that’s a bit harsh but the first half of the route up to Sanquhar is either field paths, tussocky grass/bog that is so infuriating to try and ride, hike a bike boulder paths or fire roads (if you have made it this far a fire road will have you crying for joy) with a few short but very nice single track sections so you can forgot about the first half, the route from Sanquhar is rideable and especially from St Mary’s loch onwards as there are some cracking sections of single track, most of this section was used for the 2007 Trans Scotland stage race which was a hoot!.
If i was to do it now i’d start at Sanquhar/Wanlockhead and take it from there, here is a link to Coastkids blogspot where he highlights the first section.