At any race, runners exhibit a lot of nervous energy before the start. It shows up as loud excited talking, inability to stay still and long lines at the porta-potties. The bigger the race the more nervous energy runners have. You will see people warming up hours in advance of the race. On bus rides to the start, runners all talk at once creating a din of noise.
Pulse rates reach near performance levels as people stand motionless at the starting line.
The first step is to realize that it is happening. The second step is to relax and to try to save all that energy for the race. Have a plan for what you will do at the start. It should include fluid drinking, warming up, a toilet break, stowing your warm ups and getting to your starting position. If you arrive early, instead of getting caught up in the anxiety, sit back and enjoy watching others do foolish things and waste their energy.