A few responses:
1. Various distances bring their own unique challenges. Why not stop at 800 meters or the mile? The simple fact is the mile is run far differently than a 5K or a marathon. I will agree that ultra distances run over various terrain and in extreme weather are a far different sort of test than the traditional distances. My argument is that those ultramarathons aren't worth the bother.
2. You can make a marathon 30 miles even or 50 miles or whatnot, and I see a Geb or a Radcliffe owning those races, too. But ask either one of them to run one, and they are going to decline in a polite and classy way. But their reasons will be clear though unstated. Going longer is just plain dumb. Maybe a sizable appearance fee and a big prize purse might change their minds. But for the most part, the marathon distance is the one where interest begins to wane in going beyond.
3. Other ultra distance events in cycling and triathlon have a similarly low level of both participation and competition. The winner of the Hawaii Ironman gets on the cover of the magazines. I can't even tell you the name of the winner of the Ultraman. No one cares.
4. The Ultimate Ultra is the longest event at 1300 miles. It is run on a one mile loop in New York. Maybe they should have a 1300 mile trail race such as running the Western States course 13 times.
5. From the NYT:
"In an ordinary race, a 10K or something, you're just running to race or to check your time," said Mr. Karnazes, 42, a married father of two and the president of a San Francisco health food company, as we ran on a trail west of Denver one recent morning.
But an ultramarathon - technically any distance longer than a 26.2-mile traditional marathon - is not really a race at all in the ordinary sense, Mr. Karnazes said. A day and a night of running, he said, is more like a melodrama than an athletic contest - full of euphoric highs and gloomy, dispiriting lows.
http://www.nytimes.com/2005/03/16/books/16runn.html?_r=1
Out.