I'd submit the PARAs chose people who can run quickly, not those who are performing "best". Running quickly may, or may not, be an indicator of "best", but I suspect that the PARAs wouldn't take someone who aced every single intellectual test, was more than competent in the field, and could shoot the ******** out of a fly at 3000 yds if he/she also couldn't run their PFA in less than 9 mins 45.
That's true, but being fit is a baseline for them as there's a requirement to pass P Coy. You need to be good at the rest of it* as well. They're the only capbadge I can recall for whom a middle 1/3 ranking was genuinely a show-stopper. Pretty much everyone else took from across the spectrum, with the the majority of the top 1/3 heading to combat arms.
*Mostly anyway. I don't remember academic performance making a massive dent in the rankings.