HOw much preparation did you do before hand ? I'm looking at a himalayan trip next year to around a 23k feet summit (over the course of 30 days or so). I'm taking a year to train for it, but I have no way to train for 'altitude', and as I understand it reaction to altitude doesn't correlate that much with overall fitness. You can apparently be super fit but still get altitude sick, which is concerning me.
You will suffer regardless, acclimatization just makes things possible and over time mentally more bearable experience. But prepare for 3-5 steps and rest routine in higher parts, pushing through is actually pretty stupid and will fire back quickly and badly, listen to your body.
That being said, what others say is correct - a lot of endurance training helps a lot reaching the limit of your body. Plus train carrying medium backpack uphill a lot (10-15kg).
I've camped 6000m high on Aconcagua, but I couldn't sleep well above 3000m, almost nothing above 4000m, regardless of what mild medicine/support I took. Some sleep up there like babies. High mountains are just not for me, but I am happy with European alps though, they have it all apart from that much altitude suffering. Higher peaks just for hiking below/around them like Annapurna or Everest, loaded multi week 5500m hikes are not easy neither and you actually experience way more in 3-4 weeks rather than progressing slowly up one empty valley to the top.
This is something of an enduring medical mystery despite efforts to find related genes, etc. I think part of it is the compounding of second order effects, like who can say whether individual performance one week into an expedition will be more affected by bad sleep, bad digestion, or bad headaches? If one is immune to some side effects, how long before the others really take a toll? The team member who is strongest one day may be weakest the next depending on how the schedule of different kinds of attrition and reserves all line up, so it’s really hard to predict in advance without some direct experience. Even then it’s a moving target as we train or age.
Now why the number is as it is in altitude while next guy was more fit down below, but now vomiting furiously is another question, I agree not very clear. Maybe red blood cell pace of production but even that is not a complete picture.
The altitude impact was no joke, but for whatever reason it affected all of us to different degrees. I just felt a little more winded than usual, had to take my time a bit but was overall fine, whereas one of the group had a rough go of it, needed frequent breaks and vomited a couple times. The other two were somewhere in between.
So I'm sure being fit helps, but it seems there's more to it than that.
1) cardio fitness (ability to do x amount of work)
2) altitude adaptation (ability to exist at altitude)
It takes from several days to several months to adapt to altitude, depending on exactly what you mean by adapt. The first 24-72 are the highest risk for altitude sickness (which can be life threatening).
Both factor into performance at altitude.