Adventure Marathons

Just curious if anyone has every completed one the the so-called "Adventure Marathons"?

http://www.adventure-marathon.com/

A buddy and I are seriously contemplating the Great Wall marathon in May 2014 and the Big Five Marathon in June and was hoping that there would be someone who has completed any of these to give us advice or suggestions on what to expect. We need to sign up quickly, and these are pretty expensive so we just want to know what we are really getting ourselves into. Thanks!