Yes that is very realistic. People generally improve a lot during their second year of running cross country (just from watching many people go through it). You will see that your times at the beginning of this season (fall, not summer) will be much faster than your first season of cross, but you won't make as large of leaps in the middle of the season (you will still make some leaps).
Build up you mileage to 40-50 miles per week by the end of the summer and you should be able to go low 16:xx. Start off at 20 miles per week like you have been doing so you don't shock yourself into injury. And remember to put in microcycles of increasing mileage so don't just go from 20 to 25 to 30 to 35 to 40 to 45 to 50; instead, start to increase like 20, 25, 25, 30, and then drop back down and do a week of only 20 miles for a 'recovery week' to prepare for another increase, and then go to 30, 35, 35, either drop back down to 25 or go up to 40 and drop back down to 25 the following week depending on how you are feeling, etc. Start doing hills for strength in the middle of the summer.
Make sure not to get injured. You got a stress fracture before on just 20 miles per week, so you are prone to injury. Listen to your body, it isn't life or death if you only get to 35 miles per week by the end of summer because with this amount of increase in mileage you will still improve a lot considering you've only done 20 in the past. Roll out your foot arches with a golf ball to prevent shin splints and stretch before and after you run, all the normal safety stuff