Try the A702 from Thornhill to Elvanfoot - The end is right on the M74 for a quick ride back, but I think you will want to turn around and ride the A702 back again.
It's got valley's, hills, overhanging tree corridors, waterfalls, stray sheep and wide open flat plains. There's a single farmhouse in a valley a few hunded feet below the road, a photo of which I've seen on calendars before.
I've been through in the car many times, you can easily do two miles on the wrong side of the road, because nothings coming. My Brother lived in Closeburn and rode it on a Sprint ST many times, I haven't made it up there myself on the bike yet, but will do it if I go to Scotland.
If you do this, please take some pictures !