How would I organize a marathon?
You’re asking about becoming a race director. It’s a lot of work for even informal races. I’ve never organized any myself, but there are a lot of RDs on one of the mailing lists I’m subscribed to, and they make no bones of declaring how time consuming and frustrating it can sometimes be, although obviously rewarding as well. I searched for race director mailing list at Google, and a bunch of sites came up. I’m sure you could tweak the search and get more. From the outside, I’d say that there are a few big areas in which you would have to organize: a sponsor (events are pricey to produce), the locality (closing a street is a major pain), volunteers (even a 5k requires a bunch of people to help). It’s also very impt to wheel the course and make sure that it’s the distance you say it is. You don’t say why you want to do this. In the ultrarunning