[Tman]

Like others said, take the time to load/unload the yak first and foremost.

Make sure you spend the extra $ and get a good seat, and adjust it accordingly to not what feels best but what is best for your back, and know how to adjust it while you are on the water.

Watch how you paddle and make sure you are using your shoulders and arms instead of your back, if you are doing it right you should actually feel your abs doing the work. I have lumbar issues (L4 & L5), definitely feel it after a long day and do experience leg cramps, prob due to lack of blood flow so I tend to hang my legs over the side.

As mentioned, don't try a marathon session and instead of trying to be a contortionist, learn how to sit sideways on the yak to reach anything behind you. And stay hydrated.