That's right, your mind (brain) is your enemy. It is always trying to get you to slow down, mostly when you don't have to.
I was at an invitational track meet last weekend at which two of our runners were running. Both of them suffered from their worst enemy: their own minds.
On the first day, our runner was in the 3000 meter race. The first four laps went pretty well - he was patient and ran that first 1600 meters a little faster than when he ran his best time a few weeks ago in the 3200 meters. Then, he hooked up with what looked like a great situation - three or four other runners who were running about his pace. He could hide in the pack to stay out of the slight breeze, and let them do all the work. The only problem was that they slowed down. Our runner's brain most likely sent signals out that told him he was doing all the right things. The only problem, I believe, is that that brain ignored the sensation that he was slowing down, a sensation that this individual would ordinarily have noticed in an instant. So his brain said "You're doing great! Keep doing THIS!" when it should have been saying "Dude! You're slowing down! Pick it up!"
Our other runner ran the next day in the mile. In his case, he started off at the pace he hoped he could run for the first two laps. In the third lap, he tired a bit (as he should have!), and several runners passed him. As an experienced and talented runner, he should not let this kind of thing bother him. I don't believe it did, at least not consciously. But I think his brain said, "Nice try, kid, but you're just not as fast as those other guys." The final lap was interesting because those guys who passed him were about 5 meters ahead of him, but never increased that lead. To his brain, it appeared that they were out of reach, but standing where I was, it was a gap he definitely could have closed. IF his brain would have let him. But instead, his brain (subconsciously, I believe) told him the race was over. His final lap split was the same as his third lap, even though he had the ability to sprint in with a time 3 or 4 seconds faster, which would have put him ahead of at least a couple of those other guys.
Why does this happen and what can you do about it?
I think it happens as a defense mechanism. Your brain is in charge of your life support systems, and any time your heart rate and respiration rate and lactic acid production rate start heading upward, it's your brain's job to figure out how to reverse those trends. Your brain starts to tell you that you must slow down. It tries to trick you by telling you you can't do what you want to do. Even if it is doing this subconsciously, it is really, really good at it.
What you can do about it is, first and foremost, train appropriately for the goal you have. If your goal is to run a mile in under 5:00, train for that. But the key to overcoming your brain's tricks in the actual race is to trust that training. If you're trying to run under 5:00, but you've only trained enough to run under 5:15, then this will not help. If you have done the training, the next trick is to acknowledge that your brain WILL try to trick you. When you get to that third lap, start thinking, "I know my brain wants me to slow down, but I know - because of the training I've done - that I don't have to. I can actually keep this pace up, even speed up, because I know what I actually can do. My brain is just trying to trick me."
So next time you're in a race, remember that, no matter how well-intentioned it might be, your mind is your enemy. Use that knowledge to run the race you want to run.