When you're done talking about the thing and obsessing over the thing and you actually, finally, mercifully get to the starting line to do the thing--run the race that is--the number in your head doesn't mean shit.
You'll run the race you're capable of if you can just get your big dumb brain out of the way and execute a max effort over the distance you're racing that day. You can't do that if half your brain is preoccupied with a number--either a mile split or a pace or a finish time or one of the numbers on that beeping, blinking gizmoid the size of a toaster strapped onto your chicken-bone wrist.
This is what you trained for. Now do the thing. You find out the number at the end, and maybe you'll be pleasantly surprised. You just never know. And that's why you do it.