I put this up because plenty of folks have taken this on. From this report, it is clear that after 60 or so miles, the physiological picture is not pretty and of course potentially dangerous. The real take home is that no amount of training will change out your true biological limits. After sixty miles you are no longer running in the zone, but courting real damage in order to achieve your target which is arbitrary at best.
For most folks, the actual marathon is ultimately achievable. For some of those who are over committed, 60 miles is actually doable and safe enough.
Anything more than that is canibalistic and simply a very bad idea that only a handful can handle and even then the story below holds.
Regardless, i grew up in a world in which the marathoner was a true rarity and little was understood. that is true no longer and we know that any healthy human in their prime can build up to a decent running distance. The whole process leads to a deep personal understanding of your bodies limits which leads to sound health if applied correctly..