Pika,
I totally agree, "altitude sickness aside the fitter one is the better they will perform at altitude."

AND... fitness here means not just aerobic ability but strength and endurance. It's amazing how improving strength, especially in the legs, can help endurance, once one has a decent base of endurance training to backpack an 8 hour day at medium altitudes of 4,000 to 7,000 ft. with no problems.

TRAINING BASE is a term athletes use to describe the years they have trained seriously. This means at least 5 days a week of training, month after month. As I age into my middle sixties I feel maintaining my training base is extremely important. Maintenance is all.

Eric