I paddled at the elite level in Surfboats for years and years through my late teens and 20's but now my lower back is stuffed :(
I find SUP a great Core exercise sport that I took up later in life after a lot of physio
Sadly we never looked after the core back in the day with super heavy and poor (ignorant) weight lifting techniques
I still use the concept 2 rower and try and tell anybody who listens in the gym about proper technique. I cringe when I see bums shooting backwards first and then the back and arms taking most of the strain....ouch!
Done properly its such a good overall whole body workout.