I am sure that it is accurate that the average runner (properly developed) would have 57-59 speed. I am assuming the guy meant that's what they would be capable of AFTER training, not that the average guy could run that right out of the box...
I remember high school and if you included the sprinters and long distances guys, throwing out the guys who really sucked ass (who would not be considered average I think), then the average dude was right around 60-63. Now assuming that these people were developed to their (average guy) maximum potential, then I would say 57-59 is totally reasonable. I doubt the AVERAGE guy is faster than that.
A guy who can run 57-59 in the 400 should be able to run the following times ASSUMING that the guy trained to his maximum ability (i.e. he put in lots and lots of miles for a long time, years). We are not going to ASSUME that the average guy would improve his 400m past 57-59 since that was our previous ASSUMPTION, that he would peak out at 57-59...
800m: 2:02 - 2:06
1600m: 4:20 - 4:30
3200m: 9:20 - 9:40
5000m: 15:00 - 15:40
10000m: 31:20 - 32:40
10 mi: 53:00 - 55:00
Half Marathon: 1:10:30 - 1:13:00
Marathon: 2:29:00 - 2:34:00
In summary, if you could get every average guy in America to train hard for the long distances for a long time, then the average guy would be a 15:20 5k guy. Using statistics, several standard deviations away would be a whole shitpile of guys who would be above average and running sub 13:30 for 5k. Too bad every average guy ain't out their training, huh