I have about 4 weeks until I am done with cross country. Then I am going to take 2-3 weeks off from running. Should I just build up my mileage a lot until indoor track starts? Or do something else.
Indoor track probably starts more or less right after your break. I would suggest doing whatever your coach recommends.
2 weeks is probably plenty of time off. Not sure how much time you have between seasons, but you can't go wrong with building mileage plus strides a few times per week.