In my experience, you've gone out too fast if you get to the one-mile point and suddenly go from thinking "That wasn't too bad" to "I went out too fast."
I would say that the pain should begin somewhere before the halfway point, but not by the one-mile point. It's easy to be the fool who goes out too fast and is forced to slow in the second mile and hangs on, perhaps still slowing a little, most of the way through the final mile.
I've been that fool a few times. It's always a result of bad pacing, which is to say going out to fast, feeling pretty good doing that, getting to the one-mile point, and then feeling the burn.
A 5K is going to hurt. The point is to run it in such a way that you choose when it's going to hurt, and what the psychological effect of that pain will be.
"The first half is for pacing, the second half is for racing." Who said that? The Serious Runner's Handbook, I think. That mantra helped me. Holding back a little in the first mile, settling into a strong pace that isn't a too-fast pace, and reaching the one mile point feeling as though you've executed the plan and pave the way for a good race: that's a talent worth cultivating.
Short answer: you'd be well advised not to hurt much prior to the mile-and-a-quarter point.
