Honestly there are some belting roads in this area.
Castle douglas to Ayr
Newton Stewart to Ayr
Stranraer to Ayr
Ayr to Thornhill
Try out this for a day, Thornhill to Cumnock, Cumnock to Douglas then on to Lanark, Lanark to Abington, Abington to Crawick then into Sanquhar then to Mennock and up the mennock pass to Wanlockhead then Leadhills then Elvanfoot, Elvanfoot to Thornhill. That is one good day out on a bike..