I have found a race that is annually set in Frozen Head State Park, Nr Wartburg Tennessee that is a 5 loop 100 mile race with a 60 hr cut off. It is self sufficient but you can return to your tent at the end of each loop to rest or restock.The course requires navigation and compass skills and has a total elevation gain/loss of about 58000 ft this tends to be short sharp climbs of 1000 to 1600 ft over about a mile. In 25 years there has only been 10 finishers of the approx 1000 runners. It does seem surprising the last figure as a number of well known ultrarunners like David Horton have run it. I think the problem may be that these guys don't have the self sufficiency and navigational skills that long distance fell runners automatically have. I really think some of you guys who are at the top end of fell running could really show them a thing or two british invasion style. The other issue is that you have to know the e mail of the race director and what day to send it on and there are only 35 runners a year.
If any of you are interested i do have the secret info as regards the e mail and what day to send it on. Any further info can be found on Matt Mahoneys site. Just put Barkley Marathons in Google.