The meeting point is the large layby at Loch Ken on the Mossdale side (A762). Directions can be found here https://goo.gl/maps/r7PSMwk7osz (the layby was built after the Google Car went round so you can’t see it on street view look for the biggest one near the end roughly opposite the Galloway Activity Centre).
Grade A (Coaching or Max 3km / Flat Water / Beginners)