Train hard before you go, but don't exert yourself too much while there.
Those mountains are steeper than in the east! And the lack of oxygen at higher elevation makes it more so. If your heart and lungs have been working hard training for it, you will be so much better off.
Altitude sickness is something to really consider. It can ruin your trip or worse. Don't exert yourself the first couple days to help acclimate. And remember that the only real cure for altitude sickness is to go to lower elevation. On one trip, my cousin had it so bad that he was delirious, hallucinating, and vomiting. I had to pack up camp by myself to get us off the mountain. We were able to go back up the mountain a couple days later, then I got altitude sickness, not nearly as bad as my cousin had it, but again we had to pack up camp and drive back to town and get a motel room.