I strongly disagree, it's like you are arguing that there is not a "distinction" in humans in "fast running" ones and "slow running" ones and you claim that we are all "the same". We are not the same, we all have different attributes and while for some of us "running" is a much easier activity, for others it can be quite laborious. And I'm not just talking about "fitness" levels here.

For example...
https://www.ncbi.nlm.nih.gov/pubmed/23201182

...we have explored circulating cholesterol and triacylglycerol concentrations, which appear to be decreased in hyperbilirubinemic individuals/animals,and are accompanied by lower body mass index in highly powered studies. These results suggest that bilirubin could be responsible for the development of a lean and hypolipidemic state in GS

Find an Extrovert (on the extreme side) and tell him to STFU for a whole day and see how this will work out!

[EDIT}
Upvoting for visibility