Your mindset matters as much as, if not more than your genes. Again and again people keep saying. "I did the same training as person X and he was much slower/faster."
That doesn't matter! Even if you do the exact same sessions every day of the year, it doesn't mean you've got the same mindset!
If you're doing some 800's in training and you're capable of running each rep in 2:20, but you do the first few in 2:15 and burn yourself out, or you just run them all in 2:30 and don't put the effort in, you're not gonna get the same benefit from the session as someone who actually runs the times they are capable of.
Even if two people are the exact same level of fitness, they are still drastically affected by their MINDSET.
For example, if two guys both have the ability to run the same 5k time:
Guy A starts experiencing discomfort 2km in, he can't handle the discomfort so he slows down.
Guy B pushes through the pain and maintains his pace all the way to the line despite the immense pain he's experiencing.
Guy A sprints the last 200m so they both appear to be equally as tired at the end.
Guy A has finished 30 seconds behind so he thinks "There's no way I can beat you, your genetics are superior blah blah blah".
I went for a run the other day with my brother and towards the end he was saying "I can't, I'm dead, I need to slow down." clearly if he's able to talk full sentences he's not in as much pain as he's making out to be in. This shows to me that my brother could never be a great runner because he's not willing to push himself enough. Although, it is possible he could improve his mindset in the future.
I believe one big thing that separates me from someone faster than myself is that they've got more mental strength and can endure more pain without complaint. If it were possible to measure pain numerically, if I reach level 10 in pain and then I slow down because it hurts, they might be able to get to level 15 before they slow down.