I got ninja'd twice while writing this but I'm posting it anyway.
I think you are
When combined with a number, I form a race.
Not sure, but could be the 800 metres
But combined with my position, it sounds like distaste.
H8 => Hate
Knock me over and I'm endless.
This is the clincher for me:
8 rotated 90 degrees becomes the infinity symbol ∞
But to knock me in early would be careless.
In 8-ball pool, knocking in the 8-ball before the other seven loses you the game.
I'm almost the smallest amount in here.
8 bits = 1 byte
But my value is necessary to your survival out there.
@Rubio got this one: the 8th element is oxygen, which we need to breathe
"Eight" is the longest one-syllable number