I will be a Junior on the track team next year?

I am a sprinter so i do the 100 and the 400 and hopefully i will run the 200 next year. My best time for the 100 is 11.7 and 54 flat for the 400. I only did spring track last year but next year i plan on running cross country and winter track as well. How can i improve my times for these two events? Will cross country give me enough endurance for the 400 to run a 52?