I changed my mind at least 6 times yesterday:
Will race on the singlespeed
Will race on the geared bike
Won't ride at all
Will just ride a couple of laps on the singlespeed
Will take the geared bike and see what happens
Will ride the singlespeed at my own speed, and use the "race" as an hour long reasonably high intensity ride to prepare for Colorado
Will ride the singlespeed just for the fun of it and to get used to being offroad again.
In the end, the "just for the fun of it" option won out, so I started at the back of the sports and singlespeed group and got dropped quicker than Tamar in a Devil race then spent the rest of the hour slowly catching people up and having fun whilst being passed by others taking the whole thing much more seriously.
On the final lap I caught up with Marco and we rode round togeher and crossed the line in enjoyable (if not spectacularly impressive) 92nd and 93rd places. I was 9th in the singlespeed class.