Or there were just a ton of people looking for something to do, since all I can do around here is get out to run or walk. Alone. At least I will feel better knowing I'm going to get a medal for it once a month. For me it is my only sense of normalcy left. I think the popularity was based on 2 things, 1. the theme was fun and 2. most of us have nothing better to do/people had tons of races cancelled.
That said, I thought it would sell out in maybe a few days, maybe a couple weeks. Not in a few hours.