Hi Josh - which week are you going out to Marion Reef on? (I've just been reminded you're on the same week as me)
Marion won't be only heavy GT popping. There is plenty of lighter applications available at Marion as well as heavy jigging. Of course, if all you want to do is heavy popping, then I am sure they can accommodate that!
A lot it comes down to technique so I'd suggest you take the time to practice your casting and to make the effort to get good at it. In terms of fitness, heavy popping needs a certain level of fitness to go the whole day but you don't need to be able to run a marathon. Just be sensible because if you continue casting when you're %^$$#, you'll hook a fish at the worst possible time. The casting will hurt you but in my view, not as much as the constant popping motion. There is a certain economy of movement you learn with experience in terms of the repetitive motion of casting and retrieving and until then, well, it's really hard work!
It is a good idea to have an assortment of stickbaits as these will give you a rest in comparison to the popping motion. Also, use poppers of a size which are not going to kill you.
I'd strongly suggest that you don't go out there and cast your#$!@# off early on. You don't need to do this and all you will do is compromise your ability to last the whole week. Remember that you have to last six days! In terms of what hurts first, well, that depends on each individual but in general, you will your shoulders, arms and traps really start to feel very weary as the trip goes out. Another part that really starts to die on you are your hands. You will know exactly what I mean by the time you get to about the fifth day when the popping claw forms. For me, I don't get much of a problem with the shoulders and arms. The popping claw is my worst enemy.
Getting into the gym to build strength never hurt anyone when it comes to popper fishing. This is an important aspect of the sport and good old fashioned pulling power has many times been the difference between winning or losing a fight.
Just remember six days is a lot of days. When you get tired, give yourself a break or go try some other form of fishing for a while. Pace yourself. I'm not too proud to say I'm %^$%## and that i want to catch some reefies for a break!