With mountains of any significance, you have the added complication of acclimatization to the altitude of your objective.  This is independent of your physical fitness.  For most people, this problem intensifies above 8,000 feet or so.

Adapting to altitude mostly takes time.  "Climb high, sleep low" is good advice.

Being fit, of course, is also necessary.  Living in Tucson, AZ, elevation 2,500 feet,  I could go to the summit of nearby mountains, 9.000 feet +, with a fairly intensive program that involved road cycling and running.

Superusers do not speak on behalf of REI and may have received
one or more gifts or other benefits from the co-op.