Anyone tired of me writing and talking about Aerobic Base Training? Joe Friel just posted a fantastic article about the aerobic base ride. Pay special attention to the section regarding the decoupling of heart rate versus power output. For those of you riding with power meters, this is a good way to review your workouts to see what adjustments you need to make to your training.
If you see significant decoupling at the 2 hour mark, then your endurance rides should stay in the 2-2.5 hour range until you see some improvement. If you are not seeing significant decoupling after 3 hours then you can decide if you want to ride longer in order to continue build your aerobic base or be content with your results.