Track training and workouts?

I'm a freshman in high school, 14 and starting track this spring. I'm competitive and fast but I need to stay in shape during the winter. I run 100m 200m and 4 by 100m relay (sprinter). Unfortunately I don't have a fitness center membership (ymca, la fitness, etc.) my only option is in my house and outside. My track club recommended 3miles a week. Is there anything else?Do I need a diet?