On each running number you pick up at the expo is a pen number which indicates which pen you need to go stand in prior to the start. These are based on the estimated finishing time you gave when you applied for a place. If you can't remember the time they'll probably tell you it at the expo.
If you know you've improved and can prove it with a recent half marathon time print out, they may allow you to move forward to a pen closer to the start.
That way people are grouped by pace.
As for the red start, that is mainly just people who got in via the charity places. As far as I'm aware the blue start is ballot people.