I will take a stab at it.
407K people that have finished marathons in the US. There are
303million people in the US, so roughly .14% of the US population.
Remove all the people that ran more than one marathon, and you are
getting down to at best 1/10th of a percent.
80% of the humans on earth live in poverty, many don't even have access to clean drinking water...
running around for fun is probably not high on the priority list. So, out of 6.7 billion people you have 1.34 billion that have enough
resources to maybe run the thing. 27% of those are under 14 years old and 3.5% are over 70 (yeah I know people over 70 run marathons but not
many).
So... of the 1.34 billion that could run you are now down to
93,130,000 potential runners. Lets use the US percentage (a high
estimate no doubt) of .10% of these "eligible" world runners.... you
get 1,308,820.
1,308,820 is .019% of the total world population .... I would guess that number off and is way too high.