Extreme endurance races

How about the Western States Endurance Run? http://www.ws100.com

Or Badwater? http://www.badwater.com

Both are currently active.

This one looks truly nasty:
http://www.mattmahoney.net/barkley/

http://www.susitna100.com

100 mile race in February in Southcentral Alaska. Run, bike, or ski! Big fun! It was about -10 degrees Fahrenheit at the start this year. Then the wind picked up to 25 mph in the afternoon later that day.

Try it if you dare. lol.

There are scads of "1 day" endurance events (100 miles on foot or bike or ski). The Hardrock 100, Badwater, Barkley Marathons, and the AK events (Susitna and White Mountains) are at the top of the heap in the US. The Tor des Geants in the Alps is going on right now. 200 miles of alpine hiking/running with a truly absurd amount of vertical gain (80,000' ?). The Alaska Ultrasport (or whatever Bill calls it) is worth noting. 350 or 1100 human powered miles. I find the foot records the most impressive in that one. Most folks know about the Tour Divide mountain bike race (Banff to Antelope Wells, NM), but perhaps more impressive is the full Arizona Trail race, which involves portaging your bike across the Grand Canyon with 600 miles already in the legs. The mountain bike race on the Colorado Trail held every August is in the same vein. There are also lots of stage races on foot (Marathon Des Sables, Race the Planet, etc , etc).

So yes, there is an impressive diversity of ultra endruance events out there. Few if any compare all that well to the Classic, in that most have a set course and involve road and trail.