I've been following ultrarunning for ~10 years. For athletes that competed in college, I can predict ultra times based on their best college times. For example David Riddle 5k PR is 14:38, Jim Walmsley ran 13:50ish - their JFK 50 miler times 5:40 and 5:21 are the same ratio (1.057) as their 5k ratios.
This method is very similar to predicting a "best" road marathon time by looking at a 5k or 10k PR. This prediction system breaks down under a number of conditions - there is a very steep learning curve for ultras - much steeper than the road marathon where many athletes run far below their physiological capability. Another breaking point is that many top ultra runners don't have a formal training background.
However, Max King, Sage Canaday, Jim Walmsley, Rob Krar, Hayden Hawks, Zach Miller etc.. all ran in college. Max, Sage, Jim, Rob, and Hayden all had college PRs (1500m, 5k, 10k) that are roughly equal or slightly better than 14 flat for 5k. Zach's college 5k PR is > 15min. Zach's 8k CC and 10k best times are all physiologically similar to this 5k time. Zach is now putting up race performances that are physiologically equivalent (or better) than the likes of Max, Sage, Jim, Rob, and Haygen.
Note that I stated above that many ultra runners run far below their physiological capability - this happens for various reasons. That said, is Zach Miller a singularity in that he is one of the very few ultra runners who is competing at or near his true physiological potential?
Does anyone have examples of someone who has developed from a >15 min 5k runner in college to a 14flat min 5k post-collegiate? This type of improvement is similar to going from 2:27 to 2:15 in a road marathon. I know loads of athletes who have made post collegiate improvements that would equate to 15-30 secs off their 5k time (6-8min off marathon). I can think of no examples of anyone dropping 60-75 secs (12min off marathon).
I know Zach is very well liked within the ultra community so many may be quick to defend a friend. Due realize that his case is highly unusual for someone who trained hard through their college years who was not stricken with Mono or injury plagued. Was something in college holding back his development - injury, illness, studies, etc? The ultimate question - is Zach doing something unethical (i.e. doping) or is he just a shining example of a previously under-trained athlete living up their potential by hard work?