It's an attitude, and
Choosing the attitude that helps you achieve your goals is part of the mental game - a part of sports, business and life in general.
Choose, "I am strong, I am light, I run like the pond skater skipping across the lily-pads" (pick your own phrasing) and you are more likely to endure the discomfort associated with running 30,000 strides over 42kms than if you choose the attitude,
"My feet hurt, my back aches, I can't do this!"
It is common sense really. Norman Cousins famously spoke about the language we use when talking about suffering with terminally ill cancer patients. He noted how the phrasing affected the attitudes of the patients - either positively, or negatively. Of course we should choose the positive approach every time. The point here is that our "attitudes" must per force be couched in language, therefore choose positive language. We think in words. Words affect how we feel. Connect the dots.