In reply to bigbobbyking:
Looks like you can get to Glentrool via the southern Upland way, not sure how bike able the route is, but from looking at the map looks like you can bypass some of the questionable single track sections via minor roads and forest tracks if need be. Coming back it looks like you can link up forest tracks and minor roads going via Newton Stewart, but probably more on road than off this way back.
In general most double track forest tracks on OS maps should be fine to cycle, though some might be fantastic and smooth others rough and a total bog, no way of really knowing before you go, some might be blocked with fallen trees etc but these should be easy to bypass.
Sounds like a good trip plan.