Even my longest (published) fic, The Ridulous Notion at 21,000+ words, was spontaneous. Actually I wrote most of it so fast that I thought it came out terribly and was originally going to post it under another pen name, just to see how people would react to it. (I'm glad I didn't. For some reason, it became more popular than any fic I've ever written.)
Maybe my careful, planned fics are my practice fics, which I am just passing the time with so that my writing won't turn into something dreadful. Maybe I am just a silly dreamer, always striving for the unlikely.
You know, even this post was a spontaneous one, cutting in front of another post I was going to make about recommendable fics. Go figure.