5k compared to 10k

[Jon Allen]

Assuming flat terrain, the fastest way to run a race is to even split it- that is, run each mile the exact same. 

That being said, for longer races (i.e. marathons) it is sometimes better to start out a bit conservative and be able to pick it up a bit at the end.  But for races like 5k or 10k, they are short enough that going out just a bit faster and then "trying to hold on" as long as you can can also be good.  There is no one answer, though- some people are good at hanging on, some are good at even splits, some do better starting just a bit slow.

[Sasha Pachev]

I have a hunch that a low mileage runner will not run his best marathon even splitting. In fact, even a high-mileage runner may find himself in the same category.