If your goal is to finish a century (or metric century etc.) then I think you need to gradually build up your saddle time while keeping your heart rate moderate (120-150). If you have already done a century and you want to do a faster century, then I think you need to build your aerobic capacity by gradually building up longer rides at a brisk pace (HR 140-150). If you want to keep up on a fast paced club ride or race, then you need to get the aerobic capacity built up to a decent level first, and then work on intervals (fast pace, muscles burning, breathing deep and rapid, HR 150-170) for various lengths (2 minutes, 5 minutes, 10 minutes).