Coming from the South it is as Scotroutes mentions a 5hr drive up to Glenlivet/Tomintoul, I did it last year.
If you stop in Dumfries to ride Mabie, Dalbeattie, Ae or kirroughtree it takes 1hr to get to Kirroughtree and roughly 1/2 – 45mins to get to the others.
Glentress and Innerliethen are 1.1/2hr drive from Dumfries.
Some route suggestions here; http://www.mtb-borders.com/routes.html
Take in Drumlanrig as already mentioned and then the A702, Dalveen Pass over to the M74/6 and on to Peebles. There is some good riding in the Lowther Hills nr Drumlanrig/Thornhill/Sanquhar on OS 78 Nithsdale 😀
Depending on which side of the South you are coming from ie A1 or M6 you can visit Whinlatter or stop in Keswick and do the Borrowdale Bash, Ambleside or Staveley for some Kentmere fun 💡