Congrats! I did my first century, with about that amount of climbing, when I was 25. I suffered just like you, and couldn't see the point in feeling like that. After returning to riding in my mid-50's, I got to where centuries were a piece of cake. Even centuries with 10k ft. climbing. You just have to ride what you enjoy, and if you enjoy progressively longer, faster rides, centuries can be fun. Even at my strongest, I drew the line at about 100 miles. I just didn't enjoy spending more than 5-6 hours on a bike.
Right now, I'm back to 50-60 miles for my longest rides and enjoy every time I'm out. Again - great job!