I've done it. If not for the USAC road racing rules, you could do it on a fixie, but the rules don't allow it. I suppose if you're looking at riding the 'grand fondo', which isn't sanctioned, you could ride your fixie. You will hate yourself well before the ride is over, if you do. There isn't that much elevation change, but what hills there are, are very steep. Lot's of riders end up walking the worst of the hills, pushing their bikes 'cause a 39x27 isn't low enough (for them, anyway).Johnny K said:
I did it on 25mm tires, but 28's would be better for much of the course (even the paved sections) if you could find a quality folding tire in a 28mm width.