You goal should be to improve your endurance without sacrificing speed. You can do this with moderate amounts of distance training-steady runs, tempo runs, fartlek (surge running), hill repeats and interval training of various distances.
If you are at a school that has a cross country team, it would be a great idea to join that team. At the end of the season, you will find that the training and racing you do in the fall will greatly improve your endurance. You can maintain speed by adding 100 meter accelerations to the beginning or end of your steady run days so that your 400 will continue to be a strength.
Also, I highly recommend at 2-3 sessions in the weight room per week and daily ab work. Also, devote time every day to maintaining your flexibility as the increased volume will affect that as well.
Develop a year long plan to get you to your most important track meets next spring in the best shape you can get to. Your coach should be instrumental in helping you to develop such a plan.
Good luck!
