500 miles is a long way.i do 500 in the car from South Wales to Fife, Scotland.Non stop, with no delays & around the speed limit equals a 9 hour trip with stops only being to take a piss & go again.
I'd second a stop in Moffat, it's kinda convenient as more or less the end of the Motorway slog & it's a nice place, get yourself into the massive Moffat Toffee Shop for some old school sweeties & wake up fresh the next day to enjoy Scotland.From Moffat theres the A701 - a cracking little road that takes you to Edinburgh, not far along it is the Devils Beef Tub, a natural Hollow within the hills where cattle thiefs used to stash their goods.
Also running almost parralel to the A701 is the A702, still a nice road, still to Edinburgh, however much busier with traffic as it is the main posted road whereas the A701 is signed 'scenic route'.You would no doubt miss these roads if you went direct in a day.Also that's to Edinburgh but to get to Stirling you may want to take a different route from Moffat & go back on the motorway but i cant recall as i never go wales to Stirling.