Welcome.
I see that you're in Boulder. I'm in Fort Collins myself. There are a number of centuries rides in the area, including (I think) two that start in Boulder. The Sunrise Century is in June, and has over 7000ft of climbing. Looks pretty difficult unless you love to climb. The Buffalo Classic is in September and goes North along the foothills to Fort Collins. With about 4000ft of climbing, it's should be easier, but there are some short climbs that are not easy -- the climb to Carter Lake is pretty steep. Even if you don't want to do an organized event, this would be a pretty nice route.
Nick