Train if possible on the step climber. I had to stop because of gym restrictions. I was training up to 70 stories twice a week. I was switching back and forth between level 4-10 every 7 stories. I also took two Nitric oxide capsules every morning before starting out and began those a week before the hunt. My 300 is in the 13-14 pound range also with scope. I was 65 at the time, but never got really wind.When I know I'm taking a heavy rifle, my gear doesn't change, I just deal with the weight of a heavy rifle. I pack pretty minimalist anyway so there isn't really anything that I would consider non essential in my pack.
good luck training is the key.