Mark, we did a run back in 2005 from The Mull of Galloway to Dunkeld. I will look out the notes for you, as soon as I can. It was a 2 day event, but you could do a bit of the most southern part. The roads arround the Galloway Forest were great.
Mark, apologies for the delay in coming back to you. You have probably planned your route by now but if not here are a few comments. I would probably stick to the "Rhinn of Galloway" as there are a lot of empty roads around there. We started our run at the Mull of Galloway, down at the big lighthouse and that is worth seeing imho. I can remember Duncan, Helen, Neil and Lorna were on that run and I think Elfman was there as well. We took the B7041, A716, to Sandhead and then the B7084 towards Dunragit. Then the A75 to Newton Stewart and then the A702 to Clatteringshaws Loch. From there we headed northwards, which is the wrong direction for you. From Cairnryan you could take the A77 to Stranraer and then do a north loop of the "Rhinn" via the A218 and the B738 to Portpatrick and then do a southern loop via Garrochtrie to the Mull and then back up the A716, B7084, A75 and A77 to Cairnryan. I reckon that would be 80 to 100 miles and with lunch and a lot of picture stops, that might be about enough driving for the day. We are finding on NMS runs that peps are preferring shorter quicker blasts, with less miles and more time to chat, look at cars and enjoy the scenery. You could extend another loop taking in Milton, Stairhaven or Glenluce and New Luce, but I haven't been up that track via Craig Fell so I can't comment. If you need to go further, you could go South East to the Machars and Wigtown? Hope info is of some use. Weefossy.
ps found a couple of pictures.