There is an off-road route, ostensibly based from Glentrool visitor’s centre, which passes not far from The Stables, so is easily picked up in Newton Stewart. You can then either choose to take the detour to the Glentrool visitor’s centre (making 36.1 miles) or leave that out (making 35 miles). This is the route map. (click for bigger version). This version does not include a trip to the visitor’s centre.
A PDF map and description from Forestry and Land Scotland is available for download at : GlentroolBigCountryRoute
A gpx file is also available for download : Bigcountryroute.gpx