I spend a lot of 10 - 14 hour days in my kayak every summer. The longest was 4th of July 2 years ago. I put in at 7am and pulled out at 9:30 pm. I have no issues with these long days as my kayak allows for plenty of movement and body and leg repositioning. After almost 25 years of kayaking I have figured out what works for me. As I am sure most of us avid kayakers will agree that how we kayak is as personal and varies just as much as the kayaks we use. Just don't over do it for your own personal level is the key. It's great to see this new forum topic and I am looking forward to sharing with all the other kayak enthusiasts.
It is not about the equipment you have to use,
It is about how you use the equipment you have. :D