So what are you wanting to carry on a century ride?

To do a ride of this distance then the bike should be well maintained. I know things can happen that require repairs but in theory- it should not. All I carry for rides up to 100 miles is the same as I do on any other ride. Tube- levers-patch kit and a multi tool fitted into a wedge under the saddle. I would fit two water bottles and a pump on the bike and in the pocket of the jersey are wallet- phone- and a bit of food. Clothing is the type that can be packed down and carried in the pockets or into its own pouch and carried round the waist.

Or you can go over the top and carry every tool and every bit of clothing to cater for problems- but that will require Panniers fitted to carry it all in.

alpacalypse has the sensible solution and a larger seat pack to carry the few extra's- as after you have done several century rides you will find what you do not have to carry on any ride.