This morning, on a particularly hot and sticky run, I reverted to an old psychological trick that I haven't had to play on myself in years. It's the "just keep running until you get to ________, and then you can rest" trick.
(Insert "that tree" "the white mailbox" "the next street light" and you get the idea.)
You pick some inanimate object, say 50 yards away, and promise yourself that you just need to run to that object, and then you can take a rest. When you're, say, 20 feet away from the target, you convince yourself that you can run "just a little farther" to the next target - another 50 yards away. Left foot. Right foot. Repeat.
When I first started huffing and puffing my way to being a runner (we won't talk about how many years ago that was) I used this little trick regularly to stretch my runs from 1/4 mile to 1/2 mile... then to 2 miles... and eventually to my first 5k. By that point, I was hooked and no longer needed tricks to keep myself motivated. (Well, that is, until I met the sauna that is northwest Florida. Hello humidity!)
How my legs haven't caught on to what my brain is doing, I'll never know, but it does work. I "tricked" my feet into carrying me through the second half of a 90+ degree run this morning. By the end I was so drenched in sweat that I looked like I had gotten caught in a rain storm, but I finished.
And man did that cold shower afterward feel good!