I just learnt about the Long March led by Mao Zedong in 1934-35. 6000 miles semi-voluntarily seems extraordinary with a 80% death rate. Has there ever been something similar either of a group or individuals?
I found a book written by two men who retraced the route of the Long March and collected some eyewitness accounts along the way. I haven't read it, so I'm unsure of how reliable it is (notably, it claims the march was much shorter than 6,000 miles). Here's an Amazon link: http://www.amazon.com/The-Long-March-legendary-journey/dp/1845292553