To run a 2.04 marathon you need a VO2 max in the 80s; even 2.30 needs one in around the high 60s. There will be the occasional person with fantastic running economy who has rather lower figures than these. The average VO2 max in the general population is around 45, so a 2.30 marathon runner already has a "genetic advantage". Noakes, in Lore of Running, gives the example of a keen runner with a VO2 max of 50 - no matter how hard or intelligently he trains he will not get his VO2 max above 57, and the best marathon time he can hope for is about 3.06.