yes. you can break 17 on 20 mpw.
what you are doing is a very safe training schedule. Of course, your best workout every week is your 3 mile race on Saturday--hugely important. If you are happy with the trajectory, that is great. IF you want to find out how good you can be, you need to eventually do more.
A solid, but not wild high school training schedule might include something like this, every week:
A longer run of 8 to 10 miles
One race, or workout that works on race pace, or a pace above or below it +
One more workout that works on a pace above or below race pace. (These two workouts might total in the range of 7 to 10 miles, depending on the pace you are working)
A day with sprints (flying 30s, 8 to 10 second hills)
Add in some easy days to get in the 40 to 50 mile range.
you have to start somewhere. I think what you are doing is great for a runner who is fairly new to the sport, but you will not improve forever with it.