On topic wrote:
Look into the history of pedestrianism to get very fascinating information on the long-distance racing boom of the late 1800s. There were all kinds of events at various distances, a lot of them indoor and many of them on closely monitored short tracks where people could move at any pace/rest strategy they pleased. These racers were very popular as was the betting scene around them.
6 day race eventually became a standard because like you alluded to, a week is a nice round amount of time but wasn't possible with many local governments refusing to let the event happen on Sundays (pretty much everything in the US/Canada was shut down on Sundays in this period).
1 day, 3 days, and 6 days are all still popular events as are 6 hours (1/4 day), 8 hours (1/3 day), and 12 hours (1/2 day). 2 day races exist but are less popular.
Thanks for the information!