Forgive me if I missed it, but what is the distance and elevation gain of your "HUGE" ride? Also, when is the ride?

As for changing from your compact crank to a triple, that could be costly as you would have to switch out your shifters as well. What kind of cassette are you running? It may be better to just switch that.

As for your 12 hour century, that should be more than do-able for the average person, even a beginner. I'm not super fast and I've done 12 hour double centuries. If you have worked your way up to 62 miles, then a century is a reasonable next step. How much elevation gain will there be? Everyone's definition of "hilly" is different.