In my opinion it comes down to two things
Most importantly, you have to have the body type and the natural ability. That knocks out about 90% of the population from being able to run 1 sub 5 min mile.
Now for those 10% of the general population who have the potential for running 1 sub 5 min mile, I would guess only about 2 or 3% have actually done it in their life so we are now narrowed down to 2% of the general population (talking both male and female) who have run 1 sub 5 minute mile.
Now for those 2% who can run 1 sub 5 minute mile, with enough patients, a lot of hard work, and a little bit of luck, they can get to a 10k at sub 5 pace with a lot of hard work. Now we are prob at about .05-.01% of the population who has run a sub 30 6 mile.
This is as far as I have gotten and from there I have no idea how you do 26.2 miles at sub 30 10k pace, Any elite marathoners here wanna share?