Many years ago, I was a member of an expedition that climbed a peak that was about 80% the height of Everest. After we finished and came home, an account was posted on the web. Less than a year later, an Army Lt. Col. in Special Forces contacted me and started asking questions about the peak. He planned on taking his elite military team there in about three months, and he just had basic questions. When I asked, it turned out that they had Zero high altitude experience, despite the fact that they did most military things well. I told him that they needed to get out to somewhere such as Fort Carson, CO, and backpack up some 14,000-foot peak, sleep a night or two, and then come down and report back to me on what happened. He thought I was being silly. Later, the military team went to the peak, climbed most of the way, and failed. Next time, maybe they will listen.
In California, another good place to hang out at altitude is in the White Mountains east of Bishop.