I can relate to this, in more ways than one, too. My present work, supposedly aiming for 5-6k words, is closing on 50k. How could I have misinterpreted the seed so badly? As you say, fiction writing is the most amazing process. More than anything, we need to belive in it and not question our own capabilities. The fact that we forget the names of co-workers, or what we had for breakfast, is neither here nor there.
Thanks for sharing.