Much of the US population is unfit and perhaps many talented runners are never discovered. The mean 5k road race time for men is apparently 25:00. But what if everyone was a runner? Here is an estimated distribution for a very fit population of men aged 18 to 33 (extrapolated from a combination of Japan's best 10k times and from a study of the Ache people in South America).
Mean 5k time for men aged 18 to 33---19:00
Two thirds come in between------------17:30 and 20:30
95% come in between------------------16:30 and 22:30
% WR speed--Top X%----1 in X men----200m-----400m-----800m-----1 mi*----2 mi*----5 km-----10 km
63%-------------74%------------------------30.42-----68.45-----2:39------5:50-----12:29----20:00----41:39
65%-------------63%------------------------29.66-----66.74-----2:35------5:41-----12:10----19:30----40:37
66%-------------50%------------------------28.90-----65.03-----2:31------5:32-----11:52----19:00----39:34----average young man
68%-------------37%------------------------28.14-----63.32-----2:27------5:24-----11:33----18:30----38:32
70%-------------25%-------4---------------27.38-----61.60-----2:23------5:15-----11:14----18:00----37:29----top 5 in average Ache tribe
72%-------------14%-------7---------------26.62-----59.89-----2:19------5:06-----10:55----17:30----36:27
74%-------------7%--------14--------------25.86-----58.18-----2:15------4:57-----10:37----17:00----35:24----fastest in average Ache tribe
76%-------------3%--------33--------------25.10-----56.47-----2:11------4:49-----10:18----16:30----34:22
79%-------------1%--------100-------------24.34-----54.76-----2:07------4:40-----9:59-----16:00----33:19
81%-------------------------453-------------23.58-----53.05----2:03------4:31------9:40----15:30-----32:17
84%-------------------------2600-----------22.82-----51.34-----1:59------4:22------9:22----15:00----31:14
87%-------------------------22,000---------22.05-----49.63-----1:55------4:14------9:03----14:30----30:12
90%-------------------------300,000-------21.29-----47.91-----1:51------4:05------8:44----14:00----29:09
93%-------------------------7 million-------20.53----46.20-----1:47------3:56------8:25-----13:30----28:07----olympic finals
97%-------------------------342 million---19.77----44.49-----1:43------3:47------8:07-----13:00----27:04----gold medal
100%-----------------------------------------19.19----43.18-----1:41------3:41------7:53-----12:37----26:18----world record
*Converted from 1500m and 3000m times
This spread is probably what you would expect if everybody realized his full running potential. Looking at the 'average' guy's times, they look just somewhat slower than what you would expect from a DIII college soccer player.
One way to define a 'serious' runner might be such that you have enough talent that your VO2max (or your sprint speed) could be useful to your tribe if you were a hunter gatherer. This should put you roughly in the top 3 to 5 young men in your tribe, so about at 70% of world record pace (18:00 for the 5k).
Persistence hunting with the top 3 men in a Khoi-San tribe:
http://www.youtube.com/watch?v=826HMLoiE_o
Study of the Ache:
http://anthropology.missouri.edu/people/walkerpubs/phys12.pdf
Source of Japanese 10k times: