My first century was last October and was a bit anti-climatic. I was doing an 8 day supported ride from San Francisco to San Diego. Our usual daily mileage was between 80 and 92 but on the last day three of us decided to ride all the way to the Mexican border. We started to run out of daylight so did a fast (by our standards) pace line of around 22 mph for the last 10 miles. Actually, our strongest rider pulled for the last 10 miles and I did all I could to not lose the wheel of the woman in front of me! We ended up with 105 miles for the day. I was happy we'd made our goal and finished the tour but the ride didn't seem much harder than the other days (and easier than the Big Sur section). I think I'd have been more excited about finishing my first century if it hadn't been part of a tour.
One thing I found interesting about that ride was for the first three days my legs felt worse each day. After that they started feeling stronger each day. It exhilarating when I started climbing in a gear other than my lowest! However, my butt got a little worse every day until the last day I wore two bibs and stayed out of the saddle all I could and it still sucked. If I did a longer ride I'd have to work on that issue since I don't know how many more days my butt could have lasted.