My back is not the best either..

Too many catapults windsurfing..

..actually I think the jolts of landing from jumps over the past 30 yrs of windsurfing hasn't helped either.
I think stand up paddling has helped me and it feels the best that it has for years..BTW..I get out stand up paddling almost every day now.
I think it is very important that you have a correct paddle length so that you are not bending over..I find myself very upright..with a little knee bend..and swinging both arms through an arch with almost straight arms.
My paddle is 13" more than my height...(I'm 6'4")...It may be a little long for serious surfing...but I'm mostly on flat water.
This position puts no load on my back and lets me paddle for hours at a time.
IMO..Go for it..

DJ