Another one comes from Santa Claus Is Coming to Town. You can say this to yourself while you are running. All together now "Put one foot in front of the other!"
And there's always that great line from Forrest Gump. I like to insert my name here: "Run Forrest, Run!!"
Triathlons, marathons, 5Ks, they're all long. It is a mental game of repetition - the will to NOT quit. So find your mantra and try it out. You'll go longer and faster than you'll ever think you could. And if you can't think of your own, go ahead and use one of mine.