ELI5 Marathons - What happens to the body over 26.2 miles?

Does a marathoner really get an inch shorter during the course of a marathon race? What else happens to the body when you run 26.2 miles? Why is the marathon distance even 26.2 miles in the first place?   ... We explain like i'm five!     Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: oakteach, medstudent0302, janguskhan, dyslexiaisagfit & tarpit_carnivore   BBC News articles referenced: What happens to a runner's body? http://news.bbc.co.uk/2/hi/uk_news/magazine/4259766.stm The body's marathon effort http://news.bbc.co.uk/sport2/hi/athletics/4326947.stm   To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]