It's only a ride and you did more than 99% of the population could do. Plus, now your bike will fit correctly!
My suggestion would be to try a flatter route for a century, don't try to keep up with anyone, and enjoy. I did a flattish century with no special training, other than my usual hilly rides and a few 65 mile rides. Very few. While there were no big hills, there was a vicious headwind, rain, and thunder. Our goal was to finish in 7 hours and we did it in 6.5, about the same average as you had. If you can find one person who you can ride with, at *your* speed.