Fifty kilometers (31 miles) is the distance of the longest race in
either the Summer or Winter Olympics. Racewalkers compete at this distance in the summer games and cross-country skiers do it at the winter games.
The world best performance for racewalking was done by Thiery Toutain on 9/29/96 in Hericourt, France. He completed the distance in 3:40:57, which is faster than most people can run it.
The 50K is also the shortest distance that can be called an
ultramarathon by runners, although it can be run as a slightly longer marathon. (A marathon is 26.2 miles.)
The best performance on the road was done on 4/2/1988 by South African Thompson Magawane in 2:43:38, which is seriously kicking some ass. That's fast enough to win some regular road marathons. Just a little off the pace is Great Britain's Jeff Norman's 6/7/1980 track best of 2:48:06.
My own 50K experience is much less stellar, a 5:19 over a multiple-loop, wild-boar-rutted, sandy and muddy course outside of Sarasota, Florida. I'd had the stomach flu the week before the race and was a bit woozy. For the most part, the 50K is a low-key, easy-going affair and not a bad way to spend a day.
Sources:
http://www.runnersworld.com/home/0,1300,1-141-147-1096,00.html
http://www.runnersworld.com/home/0,1300,1-51-166-4575,00.html
http://www.racewalk.com/Records/pNMRecords.htm