-Pump those tyres up and oil that chain
-If you are planning to make stops to take on food carry an emergency snack in case you miss a stop or get hungry in between, and try to eat something substantial rather than sugary. Also, start your day with a nice big dose of slow burn breakfast.
-If your route has hills, don't be afraid to just sit in a very low gear and slowly make your way up, this can really save your legs if you usually put in an effort on the uphills.
-Make sure you take on a lot of liquid, you'll need to refill your bottles at least once on the way around. I also recommend something to help with cramps or fuelling (electrolyte drink etc.) in at least one bottle.
-Set off a little bit slower than your usual pace to give yourself more for the extra miles, you can start to bring your pace back up if you feel strong towards the later parts of the ride.
-As others have said, think of it as a collection of small rides. I normally mentally partition my rides by refill and rest stops. If I'm doing a 100 miler I'll normally stop at ~25, 50 and 75 to stretch, eat and refill bottles - one of those will be a proper cafe or large packed lunch stop
Good luck!