My sophomore year of HS, I did track with no previous running experience. My first 1600 was 5:43. Two and a half months later, I ran 4:56. The following track season, I ran 4:39. The season after that (senior year), I ran 4:27. I never did more than 30 mpw during track season. I could have been a helluva lot faster had I understand Lydiard concepts of high mileage.
I'm sure nobody cares, but my 3200 progression was 12:22 (start of sophomore track), 10:42 (end of sophomore track), 10:19 (junior PR), and 9:52 (senior PR).
So basically you can improve a lot in a fairly short amount of time, even with low mileage. Maybe I could have gone 4:15/9:20 if I had put in 70mpw.