If you wanted to start with a bit of limbering up, we have the John Muir Way running from Helensburgh across to Dunbar with pretty good transport links, at least on our side of the country; and the Three Lochs Way; there's also the Southern Upland Way - you can get a train to Sanquar from Glasgow Central as a starting point. And there's the WHW of course, but that might be a bit over-populated if you're looking for something a bit more off the beaten track.
Good luck with it, my son is busting to start Explorers so he can get cracking on his DoE, unfortunately for him he's got another two years burning his shirt on a campfire in Scouts before they let him loose on a hillside.