You're right up to a point. The essence of training is to stress your body so it responds by getting more fit. If you want to ride a century you need to work some distance so you break in your butt and get your core/back used to time on a bike, but long slow rides won't make you faster and increase cardio fitness as well as intervals so my basic advice is to do work some gut wrenching intervals into other good paced distance rides that involve longer time in the saddle.