How can I prepare for cross country?

I'm a girl and I will be a sophomore next year. I did track and did the longer sprint distances such as the 300 hurdles and the 400. I was wondering how I can get in shape for cross country so I could run a 3.1 mile in under 20 minutes. (and yes I know I'm setting a high goal) but I really want to to good and I have the motivation to do it I just want to know how to train during the summer for it. Btw my brothers really fast and he would always help pace me during track so what would be a good 3 mile pace for starting off and what are some good ways to train for cross country?