It sounds like you are just letting your fear stop you. You have good working knowledge of the bike, you have said the clipless pedals help you.. So the only thing stopping you is.. You.

This is what I would suggest. Take your bike out into the country. Even if you have to drve to get somehwere where the traffic is not as bad or the roads are a bit less hilly. Do as many rides in the "country" as you need in order to get comfortable on the pedals. I can tell you this as a fellow clipless user that it will get better and the clipping in/out will become second nature to you.

Good luck and I hope you enjoy your century!