Suppose you've ridden the local century in 6 hours and 20 minutes. Now your goal is to break 6 hours. That'll take a 16.7-mph average speed.
You might find it relatively easy to maintain a 17-mph average during the first 2 hours of a 5-hour training ride. The third hour, it's tougher. The fourth hour, you're suffering. The fifth hour -- well, you're finding out the hard way that you aren't able to sustain your goal pace. Training with long, steady miles isn't producing sufficient late-ride strength.
Riding fast when you're fresh isn't the problem. The stumbling block is the ability to ride fast when you're fried. You need to train in a way that helps you overcome this "sticking point."
Try this: Do most of your next long ride (4-5 hours) at a steady and moderate pace that's slightly below your goal for the century. Keep something in the tank.
Then in the last hour, include two repeats of 20 minutes each. Ride at an intensity of about 85% of your max heart rate. The effort should feel "hard," at least 8 on 10-level scale of perceived exertion. Spin easily for 5-10 minutes between the hard efforts.
Important! Be sure you're well hydrated and have been consuming enough calories on the ride before you start these intervals.
Riding the last hour this way several times trains your body to go hard when it's tired. It'll help you overcome end-of-ride fatigue and get that personal record.
Source: RBR