i'd suggest increasing your mileage by about 5-8 miles each week, with the following:
1. one long run each week. 18-22% of your weekly mileage. example: 50 mile week is a long run of 9-11 miles.
2. day after your long run, run 5-6 miles and finish on the track. take your shoes off and run 2 miles barefoot, pushing 100m (at 400m pace) with 300m jog recovery. do a 1 mile cool down (barefoot or in shoes, your choice).
with these 2 workouts you will be working on aerobic development and speed/ anaerobic-glycolysis development.
so let's say you do your 10 miler on sunday, and your 6 mile run with 100's and cool down on tuesday, you're at 19 miles for the week thus far. that leaves you 5 days to run 31 miles, which is an average of approx. 6 miles a day. i'd alternate between 5 and 7 for the remainder of the week. as the summer progresses, add a 3 mile tempo run. a tempo run is also referred to as a threshold run. run for 20 minutes at about 85% maximum heart rate. do a warm up and cool down also. that could be about 8 miles total.
a 10 week progression would look like this:
week #, mpw, long run, additional workout:
week 01 35; 7-8
week 02 40; 8-9
week 03 45; 9-10
week 04 50; 9-11
week 05 55; 10-12 20 minute tempo run
week 06 60; 11-13; 20 minute tempo run
week 07 65; 12-14; 20 minute tempo run
week 08 70; 12-15; 25 minute tempo run
week 09 70; 12-15; 25 minute tempo run
week 10 70; 12-15; 25 minute tempo run
notice that a 9 mile long run is sufficient for weeks 2,3, and 4. however, increasing it will help you greatly. same thing with the 12 mile long run, which is sufficient for weeks 5 through 10. it may seem like a lot now, but if you gradually build up, your body will adapt and acknowledge the stimulus and prepare itself. it will learn to handle the newly acquired stress, and after 3 weeks will become accustomed to the long run, as well as the tempo runs. talk to your coach too. if your coach doesn't know anything about coaching cross country, find a coach in your league who might allow you to train with his team during the summer. he'd probably appreciate having an extra runner to work with.