There is no way that Amy Hughes did 521, 480 or 400 miles in 7 days.
She claimed that she did 60+ miles in the first 12 hours. That does not mean she actually ran 60 miles in 12 hours. For a good ultrarunner 60 miles would be feasible in 12 hours but more on the fast side, if you want to go on for another 6.5 days.
60+ miles over days is not easy for any runner. Unless you have done it, you have no idea what you are up for on day 2 or 3.
Again, if Amy Hughes is that good, she shouldn't have a problem to sign up for a 6 day race and run 360-400 miles. I believe it when I see that happen, but I doubt that will happen.