Forget all the mumbo jumbo posted so far in response to your question.
I ran in the 18 min to 17:16 runner my freshman and sophomore years (on 30-35 miles per week with lots of short intervals (400s galore)) to 15:40s the next year by simply adding 5 extra 4 mile runs at 8 each night at a steady (moderate) pace to what we were doing. I also begged our coach to let us run longer intervals. BUT, the key was the mileage difference. My senior year, I ran 15:41 the first week of school at a meet (hot outside too) after training 50-55 miles per week for 6 weeks prior to that over hilly courses at moderate paces and running one workout per week at an 85-90% effort for 3 miles (about 20-30 seconds per mile slower than all out, I figured). No speedwork was necessary to produce a good perfomance and beat a lot of guys who went on to become collegiate running stars.
If I had to do it all over again (now I am in my mid 30s), I would do double runs per day for many weeks instead of 3-4 months per year. My wife who was a multi-state champ (18:12 runner) claims that her daily morning runs of 3-5 miles were the key to her success. As a track coach, her best runners over the last 15 years all did morning runs. Those that stopped doing the morning runs were far slower the next year.
You don't need to ask your coach anything. It is your life. Yes, if you had a really good coach then you could get their input, but that is not the case, here, according to you. Look at it this way, if you just try adding extra runs for a month and don't see results, you can quit and never go back. I am certain that you can drop a minute off your time in 6 weeks if you bump your mileage by 20 per week with extra 4 mile runs and get it up to the 50s.