I do not think it is "training" as much as pacing. There was a study and the most fit did not necessarily do the best at altitude - they pushed it too much at first. The "rule" is never to get out of breath - keep your blood oxyginated. Start really slow and perfect the "rest step". For sure, being in shape helps but I think your high altitude technique is more important. Rythmic breathing is the art of setting pace based on breathing - set a steady breath rate and then surely and steadily keep your walking pace at a level that maintains your proper breathing. Sort of like gearing down in a car when going uphill. I live at sea level and go to 14,000 feet on the weekend, and do not feel the altitude until 12,000 feet. I also really concentrate on my breathing and try to never get out of breath. Also drink a lot. I also take Advil before I get a headache. I may be blessed with good altitude genes, but I have taught a lot of students at NOLS, and with proper technique, most students did fine.