I'm 52. I would just suggest possibly shorter days on the yak for you. I find that after 4 or 5 hours, I have usually had enough. I recently went out on one of those Islander trips. I spent 12 hours on the yak. I was pretty sore the few days after, but not too bad. I wish I would have started 10 years ago, but hey, what the hell. Better late than never. Keep at it, just DON'T STOP!
Janine