Sometimes in the middle of a run, your brain can be your own worst enemy. Your thoughts are overwhelmed by your joints exaggerating their levels of fatigue, or even worse, sheer boredom. These are a few ways to get you through the process.
1. Music.
Seems like an obvious choice, but it's surprisingly effective. Depending on the length of the run, it helps to have music with different tempos. Keep one playlist with a similar beat to your pace, and another with a faster beat for when you need to pick it up a bit. This probably isn't the best time to listen to anything for the first time, as you don't need to be distracted to that level. Also, you want the music to be reliable. If the next song is a down-tempo ballad about the singer's dead dog, it doesn't exactly motivate you to get moving up that next hill. Nobody has ever said "That Gordon Lightfoot song 'Wreck of the Edmund Fitzgerald' gets me so pumped! It's on all my gym playlists!"
2. Turn into "The Running Tourist"
It's as simple as it sounds. Engage the brain enough so that you're keeping your form and breath control, but keep your head up and really check out your surroundings. Take a look long look at things around you. Notice the license plates of car and try to figure out why a person would pay for vanity plates reading "2LGT2QT".
Full Disclosure: this works really well at a race, or even just running around the neighborhood. Not so much if you're just grinding out on the treadmill.
3. Reward System.
It sounds silly, but creating a little reward system for yourself can work wonders. If you, or some external force is marking off the mileage set little goals at every mile or half mile. A sip of water ever mile, a notch louder on the music volume, anything. I've even heard of someone putting the corresponding number of rubber bands on his wrist as the miles in the race, and taking one off every mile. Kind of strange, but it works for some people. Keep track of your mileage with this pedometer.
4. Straight Up Disassociation.
This may not be the best tactic for your mental health, but it will help you keep your eyes on the prize. The trick is to focus entirely on the end goal. If you're in a race, think about the finish line and getting your weird space blanket that they hand out. If you're tooling around the neighborhood, think solely about that drink of water when you get back home. Or even just about how weird that feeling is when you step off the treadmill after a long run and suddenly your surroundings change when you move and you get a slight feeling of vertigo.
5. Itemization.
Start with your toes and think about each body part one at a time. Assess how much stress each area is under at the moment. If your ankles are bothering you, check in with your knees and make sure they're feeling ok. If your hips are bothering you, think about your hand position. Don't clench your fists. Kind of like a mental hot potato of pain.
6. Odd Thoughts.
Sometimes it helps to just let you mind wander and think about things you would normally categorize as being low priority thoughts. Like silly things you could use as small talk at cocktail parties. Even if it's made up, things that are conversation starters help. "Oh, I don't need to work. My grandfather and his uncle invented the straw."
"Did you know the name of the things on the ends of your shoelaces, "Aglets" are named after inventor's mother, Agnes?" (Completely made up, by the way).