There was a time when I couldn't even imagine running for 5K non-stop. Now I can't conceive of running 42klms - running or any other way!! I know how long it takes to drive a car 42klms!!!
I once watched a video of an interview of one of these 2hour type marathoners. The fawning interviewer said something like "It must be very hard for you to run the marathon in 2 hours" The runner said :" Oh no -- not hard for me at all. The runners who do it the hardest are those who run it in 6 hours - I don't know how they do it"
I can't even conceive of walking for 6 hours - or even standing up for 6 hours. So I went on a search for other 6 hour runners -- because I know that there are many of them - and found tha there is info like this marathonbasics.com/content/...