That some people are just waaaay more genetically gifted for endurance than others and there’s almost no way to close that gap. All that fast twitch / slow twitch stuff we learnt in school plays a huge part.
And I can’t help but be salty that my brother in law can run 2-3 times a week tops, take actual months off, never worry about zones or anything and still be faster and fitter than I will probably ever be - while I’m consistent, deliberate, log more miles, weight train and in better overall shape than him… doesn’t matter lol
I completely agree with you!