One way I have found to ease my mental strain is to run with others. Having someone else there pushing you along helps me tremendously. And, if you look over at your partner, and you're BOTH dying, then I think it's safe to say that you can take a break without beating yourself up about it.
While running, I recite multiple positive phrases to myself in order to push myself farther and faster. Some of my common ones are "you can do it!", "don't give up now!", and "think about how good you'll feel after!". Also, I have begun to tell myself that I am not finished running until I'm done. Meaning, I haven't reached my end point, so there is no option to stop. So, since I can't stop running, I might as well stop thinking about it and let go and enjoy it!
NOTE: I do not think walking or taking a break is giving up!! I take multiple breaks and walk. I believe that a person knows deep down when they have given up and could have tried harder, and it really comes down with how you feel you performed.
And, if all else fails, watch this video and this kid should get you pumped up and ready to achieve your goals: