The two most important things to determining how fast you go on race day are oxygen and fuel. It is a little more complicated than that, but how your body uses oxygen to convert fuel to energy is really important. That said, having the correct combination of nutrients and calories necessary to make that happen is critical to our being able to train and race well. Check out these articles on the topic.
"Fueling a Busy Fall: Nutrition for the jampacked schedule of a high school runner."
"Good Diet for a Track/Cross-Country Runner"
"To Thrive, Many Young Female Athletes Need A Lot More Food"
"Eat up! Girls require steady fuel to thrive"
High School Girls: Are Your Ferritin Levels Up to Speed?
Iron and Ferritin and Their Importance for the Female Distance Runner