Thanks everyone. I'm feeling a bit better today, but not 100%--and it's about to rain again. So it looks like I'm taking Road Raven's advice to take it easy.

Yes, it's the Healdsburg Harvest Century. I'm from Napa, so I know what the weather can be like. I'm planning to get an early start to beat the heat (although I'm driving over from Napa--I'm a morning person, but not necessarily an hours-before-dawn person). I've never really ridden anything truly flat. Around here, even the flats are slightly uphill (downhill on the way home). I wonder what that will do to my average speed, not having that downhill boost when I'm tired.