I recently completed my first Century which has been a goal of mine for a couple years now. It was an awesome experience!
Leading up to the century I was slacking in my training rides and after a few cramps on ~50 miles rides I wasn't sure if I could finish all 100 miles. At some point I just started telling myself I was going to finish despite having no idea if I was going to or not. I dialed in my nutrition & supplements - Rapid Rehydr8 (salt/electrolyte), Endurance360 (Caffeine + STUFF) & Sports Legs (reduces lactic acid production). I highly recommend Rapid Rehydr8, the other stuff could just be an expensive caffeine pill or placebo but I have no regrets. I tried to stick to real food over tons of gels - uncrustables, cookies, trail mix & fruit was awesome bike fuel. My bottles had 25oz of water & 25oz of gatorade, I had zero hydration issues.
Having my first century be a fully supported ride was amazing - the route was well marked with spraypaint on the road at every turn, there were marked vehicles that we spotted every few miles in case of an emergency, bike failure or simply if you had a question. There were 4 rest stops every 19-26 miles, arriving to one felt like nutting and really helped break up the ride. The route was hillier than I hoped but completely manageable til the third leg - it was 18 miles of straight rolling hills which was a real *****. Unfortunately that leg of the ride was the only time I started getting buzzed by trucks but thankfully I didn't get coal rolled. For the first 50 some miles I was terrified of climbing out of the saddle, because that's exactly how my cramps appeared on my training rides months earlier. I really had no choice during the hills and I was shocked to find that my legs had tons of strength left and climbing out of the saddle was the way to go.
We finished with a respectable 15.5mph pace and I did not hold my buddy back at all. He peeled off a few times during the ride but I don't think he ever learned how to gear efficiently so I was always able to catch up. I did have a few near misses with chipmunks & squirrels trying to kamikaze my front wheel, also my buddy spooked a deer which decided to sprint across the road approximately 6 feet behind me. Otherwise, it was a calm ride through scenic, rural Michigan surrounding by tons of other cyclists helping push you to the finish line. I highly recommend the Apple Cider Century if you're anywhere near Lake Michigan.