I have very little SUP experience, but surf with quite a few folks who do SUP now. You would want to stick with small surf and riding swells on a board that size. At your experience level don't take a 12' 6" board into a surfline up with other surfers. Practice catching small waves and swells before they break in an open area.
It's not going to be optimum for riding waves and learning control but you can learn to catch waves and stay upright. On a board that long you will be able to catch very small waves. On fast , walled up surf you are not going to be very happy.
Full Size Sail Rig