A few week's ago I read a a paper entitled "Modeling: optimal marathon performance on the basis of physiological factors".

I found the topic incredibly interesting. In this paper (from 1991) the author examines the limiting factors in human running and tries to build a model that gives you a bound on how fast a human could run a marathon. The results he got is 1:57:58.

You can take a look at the paper here http://fermatslibrary.com/s/modeling-optimal-marathon-perfor...