C'mon...Jon; not just marathons but distance running in general, i.e. "endurance exercise." Genes influence the body's ability to extract O2 from the blood, i.e. "cardiorespiratory fitness" or better known as VO2max. The point being VO2max is an important variable for distance running success (who would have thought. Lol).
So some people aren't suited to marathons? Well it doesn't take a genius to work that out does it?
As for trainability of VO2 max, the whole area it open to misinterpretation. Can we talk absolute V0 2 max instead of relative?
Your ability for your muscles to extract oxgyen is a fundamental biological process, without which you can't metabolize the food you eat. Think about it before you reply. Think carefully and logically instead of recycling the same silly old dogma. Cosider that mitochondria are essential for cell respiration. And consider that people who talk about mitochondrial density are just saying stuff they think sounds impressive. It's pretentious nonsense.
Essential cell funtion is not something that elite ahtletes have and non athletes don't. If the education system could get past such reductive nonsense, we might be on the road to enlightening people instead of filling their head with reductive pseudoscience.