1989Pre 

I just did my first, recently, and I put some of my recovery drink powder in my water. I think it helped. I used Source of Life Energy Shake by Nature's Plus, and a pinch of sea salt.
Long sleeve shirt was critical, because I have fair skin and do not use sunscreen.
My biggest lesson learned is to bring a cue sheet that has large-enough font that you can see it while riding. Have it accessible. I got lost on my 103-mile century at mile 98! lol Did 14 "bonus" miles, as a result.
Do not rely on arrows painted on the road, which you could miss or mis-read while under fatigue or at-speed or both, and do not depend on course marshalls who should direct you, but may stand stupid because they might mis-judge you as a rider not connected with the event.
Try to get a 70-miler in before the event. Bring a tube, spoons and mini-pump. (I brought some first aid gear).
Another good reason to ride behind another rider (or group of riders) that is doing your pace is that more-than-likely, they will be familiar with the route, so you won't have to refer to your cue sheet every 15 minutes.