Doing the 31 hours of July last year meant learning to deal with riding in the rain, although it was only for an hour or so. I agree that once you're wet, you're wet and it's not that bad - not really any different than being soaked with sweat. It's the getting wet part that takes getting used to. The only thing I really hate about riding in rain is wearing glasses.
The morning temps will be hard to predict so bring options with you to make a game-time decision. Personally, I'd bring short and long sleeve jerseys along with a packable rain jacket that you can stuff in a jersey pocket (or hand off to a sag wagon if there is one). Similar for tights and leg warmers - bring both and decide at the start.
For me personally, it's all about keeping my feet warm. If my feet are warm (not necessarily dry), then I'm warm. So I'd wear wool or smart-wool socks. I'm not sure it would be cool enough for any sort of covers.
Fenders might not be a bad idea if you're bike can take them (my road bike can't).