Just read this on the Runner's World website...
"To achieve your running goals, powerful legs and big lungs aren't enough—you also need a strong head. Doubts and distractions can derail your attempts, but a well-chosen mantra can keep you calm and on target. "Repeating choice words whenever you need to focus helps direct your mind away from negative thoughts and toward a positive experience".
Perhaps unsurprisingly my mantra is RUN LIKE 'ELLE!
You can read the article in full here