My best advice:

Since you've invested 20 days of prep into what feels like a short story, go ahead and keep going with the plot at hand.

Because:

1) You don't know if the story grows larger once you write it in November.
2) As you say yourself, you can write longer short stories in November to make it to 50k words.
3) Everyone is feeling a version of this, so don't sweat it.