Top endurance challenges

http://www.findingdulcinea.com/features/feature-articles/2008/september/The-World-s-Five-Hardest-Races.html

http://www.cnngo.com/explorations/escape/worlds-toughest-endurance-challenges-152211

http://www.forbes.com/2005/03/30/cx_ns_0330feat_ls.html

http://www.toptentopten.com/topten/most+challenging+races+in+the+world

http://ashotofadrenaline.net/8-most-insane-endurance-events-in-the-world

http://listverse.com/2010/04/13/10-grueling-endurance-events/

http://jonny-smartblog.blogspot.com/2012/06/top-10-hardest-endurance-events.html