Well, Mother Frangelico explained to me that a "Nun Run" is not a type of marathon. Well, not the kind you run in anyway. So the good news is that I don't have to go into serious training for it.
Basically, a Nun Run can be anything from a day-long visit to several communities, or it could be maybe a week long or so, visiting different communities, maybe one per day, especially if there's a lot of travel between them. Sometimes they stay overnight at one place and then another. So it's all in how it's set up.
I'm kinda sad I won't be getting my running shoes out again, though. But happy my rosary will stay active!