When the gun goes off, no matter how many times you've raced, you take a step into the known. All you are expected to do is your best, and that is exactly why it is so intimidating. The pain of doing your honest best is intimidating - you will have to push yourself to your very utmost limit. Facing the possibility that your honest best on race day may not be what you hoped after all the work for is intimidating as well.
One thing that helps me is putting the race in the proper perspective. I learn from Helaman Warriors, of whom Helaman said:
Now they never had fought, yet they did not fear death; and they did think more upon the liberty of their fathers than they did upon their lives; yea, they had been taught by their mothers, that if they did not doubt, God would deliver them. -- Alma 56:47
I also learn from the words of Shadrach, Meshach, and Abed-nego:
If it be so, our God whom we serve is able to deliver us from the burning fiery furnace, and he will deliver us out of thine hand, O king. But if not, be it known unto thee, O king, that we will not serve thy gods, nor worship the golden image which thou hast set up. -- Daniel 3:17-18
Those examples teach me how not to fear or worry, but approach the race with faith. I thought I had the words to describe this process, but I just cannot find them. Words are so inadequate to describe faith. One thing I can say is what helps is learning to become a peace with the idea of taking an inspired step into the dark, and letting God pick up from there.