"Ride faster."

I definitely agree with riding intervals. From my understanding, going long distance requires you to develop different fibers of muscle to behave in a slower manner, and interval training will allow you to strike that nice balance between stamina, strength, and endurance. Going out and riding as hard as you can is a great way to break down the muscles to build them, but you do need to take the time and test them in different mannerisms for flexibility of routine.