Hello Dan Kuss,
Have you paid attention to the form you use while doing these activities? Do you know if you are shrugging your shoulders? More than four hours is a long time to be doing anything.
I also like Calvin Yeong’s detailed description.
You will most likely get the best results by getting checked out by a doctor who can point you in the direction of a body and movement assessment.
Also, have you looked into taking kayaking lessons for the proper stroke form? What works for me is to paddle by twisting the torso left and right so the front middle of the life preserver moves. Do not paddle with mostly the arms. While paddling, push with one hand while pulling with the other.
Do you sit far from the steering wheel while driving? How is your form behind the wheel? This is not your fault, as seats are not made with the best human function in mind. Maybe you could have someone take a picture and/or film of you while doing these activities.
I hope this helps you and you get results soon.