I have run a total of 8 marathons ever. The first one I did in my local park, just covered the distance, just to check I could finish in time to get a plane to another city.
Then I did 7 in a week across Australia. It triggered my emerging connective tissue disorder.
Anyone with a reasonably able body and 6 hours to spare can do One. It is 100% mental.
Although some people say it helps to be a bit mental too.
I have posted my turtle.