to run a 5k race, you need to substain 95% of vo2max. (elite runner lactate threshold, mostly by training)
you need to have a vo2max of at least 80 and an elite running economy of 64 ml/kg/min running at 20 km/hr, then on calculations :
80 x 95% = 76ml/kg/min running at 23 km/hr for 5k. (8ml/kg/min for every 2 km/hr and 23km/hr for 5k is near sub13 speed)
basically you can have another set of vo2max and running economy, but at least to have that kind of numerical separations.
theoretically, a fully trained male to have that kind of 80 aerobic value is around 1 in 100(rough estimations), at the same time fully trained to have that kind of RE is around 1 in 100
simultaneously to have that value, 1/100 x 1/100 = 1/10000
one in ten thousand!!!!!!!! imagine how many healthy individuals never realize their full potential
but i wonder why Japanese does not have a few runners to run into sub13 5k