I have ordered one of those front facing snorkels so I can swim and not worry about the breathing so much until I do the course. At least that way I should have the upper body resilience to not get so physically tired while learning - though am aware how mentally exhausting I find learning to be.
Good, that's the important thing.
I went back to swimming a few times in my life, once after doing London-Edinburgh-London and another time after marathon training.
Both times I was fucked after no more than 400m of swimming. Second time in the pool I managed 800m. Third time 1200m. Etc, etc.
CV fitness is great, but nothing beats time in the pool doing actual swimming.
tl;dr Turbo training does fuck all for the shoulders.