Morning Leefer,
As suggested, spend as much time hunting as you can. Sometimes you won't find the elk for 3-4 days. This year we are going for three weeks, 4 days to acclimate to the altitude and two weeks or more to hunt.
I agree with Rob and shoot 11gpp but it is more important to shoot an arrow that you shoot well and have confidence in than a specific weight.
For many years we have cut and packaged our elk in camp, taken it to a processor for freezing and then brought it back in coolers. If you take your animal to a processor and it is vacuum packed figure your cooler needs by this formula - a cubic foot of cooler space will hold 35#s of vac packed meat. If you DIY with plastic wrap and freezer paper you may need less space. Last year our three bulls had an average yield of 145#'s of trimmed and packaged meat. For the first time we trimmed and cut our meat and put in gallon ziplocs and then repackaged after we got home. We also had a freezer and generator along that we ran pretty much all the way home.
Take your time and don't push yourself for the first 3-4 days. Give your body time to acclimate to the altitude. Make sure you drink a LOT of water - keeps leg cramps away and gives you more energy. Two things help with mild altitude issues (headaches and low energy are common the first 36 hours for me) - Ibuprofen for the headaches as well as extra calcium like Tums, etc.
Best advice is to have fun and enjoy the wonder of the mountains in September, it truly is a magical time.