With my experience, I too of the opinion that it is not just the fitness that matters it is the body physiology at ALTITUDE, how it responds to altitude, that is crucial in any high altitude mountaineering expedition.
I was on Everest North Face during 2015 April-May at the age of 62yrs!. Before deciding to try Mt.Everest, I too was intrigued as to how I can translate my fitness at planes to success at high altitude. I am a Crossfitter, 4.30 marathoner, frequent trekking and climbing in Himalaya upto 20000′, marathon at Antarctica, Everest Base Camp Marathon et la. But climbing above 7000mts was missing in recent yrs though I had reached that altitude way back in 1992. I met few veteran mountaineers, doctors to get that idea and feel confident that I am fit enough to think of Everest Summit at 62yrs!. Nobody could give me exact idea as to how I can relate my fitness level to mountains.
I was doing very well during acclimatization walks, ice craft, stay at BC, at par with youngsters…..but when I started my trek to ABC (24th April) I was feeling dehydrated, breathless, weak in my limbs and retraced my path after 5hrs trek and came back to BC! Was planning to try again on 26th.
But it is a different story that we were struck by an earth quack on 25th and we had to abort our expedition on 30th.
Still I am bogged by the question as to what is the equation between fitness at planes to fitness above 7000mts, how far it would be feasible for me to plan it again.