Am I the only one who's going into this completely unprepared??

Okay, so maybe 90% unprepared. I'm writing a story that I began roughing out almost 20 years ago, and originally wrote on a 5-1/4 inch disk. (God, I feel old). Obviously, the 5-1/4 disk didn't make it through my multiple relocations, and I couldn't get the text off it now if it had, so basically, we're going from scratch and a swiss cheese memory.
Needless to say, the plot is simplistic, the characters total MarySues, and I have no idea how it will flesh out. BUT!! I have the beginning, and the climax! Whoot!!