I have run many races over the years from 400m up to 100K, and can tell you that time prediction calculators don't really work past the marathon distance. There is a very linear relationship between times and distances for more 'normal' distances. E.g. My 3000m time is my 1500m time doubled plus 5%. My marathon time is my half time doubled plus 5%. This holds true for most people.
When you start talking about running 40, 50, or 100 miles, the 5% rule goes out the window, growing to 50%+ depending on the distance. At those distances, the time increase with distance becomes exponential, with wide variances depending on the day and the person.
No, 7:12 is not fast in absolute terms, unless you're a hobby-jogga like myself. But for an ultra-distance, that is insanely fast. And FYI, Dean K. is nowhere near fast enough to approach that record. Perhaps Scott Jurek (in his prime) or Ian Sharman today could crack it though.
I would challenge any of the nay-sayers out there to go out some weekend morning and run their comfortable recovery day pace for as many hours as they can hold it. Can you go 5 hours? 10 hours? If it's as easy as you think it should be, perhaps you should enter a local ultra, just for kicks. You never know, you might like it.