Ten affirmations for running — for the third mile, the hill you dread, and the part where your legs argue and you keep going anyway.

Running is largely an argument you have with yourself while moving. Your legs are usually fine. What gives out first is the negotiation — the voice offering a perfectly reasonable case for stopping at the next lamppost.
These ten affirmations are for winning that argument, and they are deliberately about strength rather than speed. On a hard run you do not need to be fast. You need one short sentence you can repeat in time with your feet.
Pick one line before you set off and use it as a cadence: two or three words per breath, repeated. Runners have used mantras for decades for exactly this reason — a rhythmic phrase occupies the part of your mind that would otherwise be building the case for stopping.
Save it for the section you know is hard. Most people know exactly where their run gets difficult — the hill, the last kilometre, the fifteen-minute mark. Deploying the phrase there is more useful than repeating it the whole way.

1. “I have a strong and capable body.”

2. “My muscles are strong and resilient.”

3. “My mind is sharp, my body is strong, and my spirit is unbreakable.”

4. “I feel my body growing stronger and I am grateful I never give up.”

5. “My body is a strong machine, working hard for me every day.”

6. “I am confident in my body's strength and power.”

7. “I have a strong, capable, and powerful body.”

8. “I am strong, flexible, and healthy.”

9. “My body is strong and capable.”

10. “I am strong and can easily do any exercise I want.”
One short phrase, matched to your breathing — three or four words is ideal. Long sentences fall apart above a certain heart rate; a rhythmic phrase does not.
They are widely used by distance runners because they occupy attention at exactly the point discomfort would otherwise dominate it. They do not remove the effort, they change what you are listening to while you make it.
"My body is strong and capable." Present tense, no promise about the finish, and short enough to fit one exhale.
Shrink the target — the next lamppost, the next bend — and say the line at each one. Almost nobody talks themselves through a whole remaining distance; they talk themselves through the next hundred metres, repeatedly.