The coast road is a nice slightly undulating road, pretty smooth and has no tough climbs. Obviously with it being a coastal road expect more wind. Around the Sandyhills area the road narrows quite substantially so be wary here of crazy motorists. In normal traffic conditions this road in my opinion is pretty hazardous, for this reason I like to head out before 8am when traffic is sparse. Bypass dalbeattie, and head up the A745 quarry road and on to Castle Douglas. Cycle down King Street and head out the A75 turning off for the Gelston loop returning back to Castle Douglas, it's fairly flat here but nice and scenic. Head off on the Dunmuir road. Traffic from here on in is very sparse, and the road will narrow considerably looking more like a cycle path as you cycle up into the hills. There's the odd sheep dotted around, so don't go racing by them at speed - they tend to run out in front of you.
The reason to leave early ...
Slightly detouring from the route above, from KPD go straight ahead at the next junction over sheep country heading for Shawhead.