Maybe it has to do with your core strength!
How important is core strength for running?
More important than you think. When you have good core strength, your body literally performs better at daily tasks, workouts and even just walking. Good core strength starts with the strong muscles of the abdomen, back and gluteus, hip and other muscles in this trunk area. New runners especially need to add this to their program to get fitter and enjoy the benefits of running more.
On a single run your body depends on these muscles to keep you in good form and protect you from injuries that happen from bad form. If you don't have a strong core you're not able to generate a good foot return when you want to go fast or long. Your body is already tired.
Core strength on your long runs...
It's important to have good core strength or your posture will fail you. Running long distances and feeling tired, can cause you to slump over because of weak muscles in the core. Believe it or not slumping over even slightly increases pressure to joints and parts of the body that are weaker to running. Hence your pace will slow from fatigue by using muscles that don't need to be used causing you to quit sooner or run uncomfortably.
How do you strengthen you core muscles?
You strengthen these muscles by following a good program. The Stretching Institute has a program that comprehensively gets to the core...A good Core Training Program you can do in 15 to 20 minutes 3 times/week! Or order the handbook now, which includes a variety of stretches for running and strengthening exercises. Real pictures of people doing stretches help you find that it is easy to read, easy to understand and easy to follow!
The Stretching Handbook has undergone rigorous reviews by some of the most respected professionals in the health and fitness industries, receiving outstanding endorsements from the world's sporting elite.