Is there somewhere on src to find a list of marathons that haven’t started or ended yet?

There isn't on SRC, since there isn't a "centralized" hub for this. If you want to just look at marathons, check out http://horaro.org This is basically the default calendar for everything speedrun marathons

If you wish to find out about them and join a marathon, an upcoming website called oengus is not only a resource for people making marathons, but also a nice index to find marathons. Find it here: http://oengus.io

It might be worth a shot to check out this SRC page, though it's only for obtaining a post on the Tournaments forum and is by no means comprehensive. People who submit marathons there do provide general info.