Three friends and I summited Vulcan Misti in Peru (~19k ft) over 2 days with no altitude training. All of us came from sea level, had 2 days to acclimate staying in the town nearby (~7k ft). This was a year out of college, all of us were D3 athletes so reasonably fit but probably not considered elite level fitness.
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.
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.