I think that amount of physical exertion on ones body is damaging to joints, tendons, ligaments and bones. Ultimately it can be done "safely" but be prepared to suffer some problems in the future (especially runners knee). This may increase ones mental capacity to break through plateaus on the positive.
Another point for consideration is, bettering this performance level on a regular basis to maintain and improve physical endurance. It isn't logical to run 100miles every week or every other week, so in a sense it is pointless unless it was a one off event.