Never run with a pack on. Wear 2 lb. Train as often as possible by skiing or hiking for all day, weekend or longer trips. If you want to be good at climbing big mountains with a big pack on, start on little mountains with a small pack on and work your way up. Conditioning by climbing is the best thing you can do.
Heavy packs tend to tire climbers the most.
Be prepared by training ahead of time and you will do fine. Don't expect to get used to wearing a pack while you are on the expedition. Be mentally prepared for the expedition. Know before you start the climb and accept the fact that at times you will be uncomfortable and that your body is going to be uncooperative. If you don't have the ability to do this you won't be successful.