Break a sweat. Your heart rate should be elevated. You should feel that your muscles are in a working state. Stretch some more. Finish the warmup 5 to 10 minutes before the gun.
Crowded starts are a part of cross-country and distance track races.
In training, you can practice running in close quarters with teammates to simulate a race. For the actual competition, you can't be too aggressive at the start and risk burning out. But you can't hang back too far, either, and risk being unable to work your way through the field. Try to gauge your position by familiar opponents. Key off runners you know who are at your level.