If you ask me for a 5K you should be uncomfortable almost the entire time.
I don't agree 100% here. I think the first mile, even if its a fast one, usually doesn't feel uncomfortable (a little adrenalin goes a long way ;)) During the second mile is when I start to feel concerned, thinking, “maybe I did go out too fast?” (this is when the discomfort starts for me) And then by the third mile its just pain, and trying to hang on. Thats one of the things I like about the 5K, for me at least, the pain is only lasts a few minutes.
But now that I re-read the above quote, I guess I do kind of agree, there is some discomfort for most of the race ;D