Don't worry, you'll be just fine...
In my opinion, you really need to pay attention to any discomforts immediately while training on your longer rides, for instance, if your feet hurt after 50 miles they'll be killing you after 80 or 90, if your hands go numb at 50 they'll be completely dead at 70...you get the picture. So do whatever you can to make yourself comfy and if you plan on buying new shorts, shoes or gloves for the event don't wait untill the big event to try them out...there's no going back once you're on the road.
My last Century went really well except for the fact that I was used to riding in 60-70*F Temps, the Day of the Century turned out to be 90*, so you can't anticipate everything....the soles of my feet started to BBQ from the combination of the heat rising from the road and black shoes...
Simply find a pack that is riding at your pace, stick with them and have fun...remember they'll all probably be suffering as much as you are at times...eat and drink plenty so you don't bonk....
I always get so sick of Accelerade, Energy Bars and Gels by the end of the ride...๐ but they really help, eating and drinking enough is vital...
Good luck and have fun!!