I stayed in Kirkcudbright a couple years ago. IIRC we did a loop out towards Sweetheart Abbey along the coast and back along NCN 7 on one day, and the North out Towards Gatehouse of Fleet and into the foothills on the second.
Lots of good places to ride round there. There's also an option to do an off-road (gravelly Sustrans rather than full singletrack), on road loop out to Glentrool if that takes your fancy, by following the two options for the Lochs & Glens South Cycle route.