Getting through any one of the stages of an Ironman (or heck even a half of each stage) represents a level of fitness most adults do not have and will never attain.
It took for me 21 months of training to go from effectively zero to a half ironman and the biggest barrier was swimming, which is very technical in nature. I still can’t swim very well. I was in no shape at all and overweight. If you look at the speeds required to stay within ironman time limits, they are quite low and a person in a slightly better than average condition would be able to sustain them with relative ease. The barriers might be swimming technique (which one might have from training as a kid) and things like biking position and running technique but not necessarily fitness as such. In fact, I see quite a few people visually in much worse shape than I am (i.e. fatter) doing long IMs and beating me at shorter distances.
Just as the ability to learn how to fly planes within a year doesn't change the fact that most people aren't and won't become pilots.
I think my marathon was a 5:30 or something like that. Lots of walking, but I never stopped moving.