GrannyGear

Good stuff above. Start early so you don't feel rushed and can Stop whenever you like. Set a conservative pace for yourself and don't, lemming like, pick up more speed than you want because a paceline flies by. If possible, find a like-minded/like-paced group or partner. Non hammerheads can be very supportive on long, organized rides. Things go better when you can converse at times. You're riding for fun and to explore this new notion of "century". Any finish, even if you come in at dusk...or in the back of the SAG wagon after 75 miles....is an achievement well beyond 90% of your peers.

You already know your butt will hurt, your back grow stiff, etc. so smile when it happens...most everyone else is probably feeling the same to some degree. Sometimes your attitude and confidence begin to wear out sooner than your body. Stay as positive as possible. You know the usual stuff about eating, hydrating, stretching on & off the bike. You'll go a long way and, finish the 100 or not, you'll be soaked in endorphins as well as sweat, perhaps have a new goal in cycling-- or at least a clearer sense of your style of riding for now.