@asker of things, Romny was more or less correct about 20 secconds knocked off the 5k reccord.
Don't get me wrong, I know that breaking world reccords comes down to hundreths of a second even for the 5k, it took Kipchoge a whole year to bring his marathon time down by only 1 seccond!!!
But to put to scale your request that the whole country of america (population 3.29 million) switches over to a gigantic running army, (the populations of china's army is 2 million). The size of the largest most successful high school cross country teams are only under 200 participants.
In america alone, there are aproximately 92,683 Kenyon BORN americans.
What im saying is, is that you've got yourself the largest most comited team -- a super army with one goal. With all that being said, yes, the world reccord WILL definately be broken by way more than 20 secconds. Yes, Its going to be a challenge, but son, you've asked for the super soldiers to do it.
Now to answer your original question about the national 5k average. First off, the training schedule you give to your super army is crucial. Are you going with the 4 year highschool schedual? are we going with the handy dandy NCAA 6 year super speed program?
These programs will get athletes fast, fast. This means that there is a limited amount of time to get your cross country team of 329 million to be as fast as possible. But when you take away the time limit and let them grow into healthy 30 year olds (training since a child) than their capacities expand drastically.
Every american by the time they reach their 30s will actually be more capable of being faster than kids who are in high school.
Look at any super star's times when they were in high school and their times after college and they differences are insane.The problem? People have to get jobs and pay for stuff. But if on the other hand every human in american trained with top college distance programs, until they were 35, any one of them could be wicked fast.
So my guess is that the average (50 percentile mark) would land on 16:15 flat. And i'm being more than serious.
Because you can always push your self to be faster than you think, and give decades of traning, your army's slower guys can definately get close to 17:30s pushing the spread mark up and up. And when you think about all the guys who are allready fast, by the time they get to the middle of college, all of the americans who are even remotely athletic will all be under 15:30s. S
So its not hard (for me at least) to imagine an america where the national 5k 50th percentile is 15:15. Bear in mind for this estimate, physiological limitations are not correlated with physical limitations.
5k average with physiological limitation: 16:30
5k average without physiological limitation 15:10