Last year I ran 17:52 parkrun, 36:40 10K and a 1:21 half (on 50 mpw and lots of workouts)
After 6-8 months of injury I started up again and capped my mileage at 30 mpw, but I tried to race myself into fitness by doing parkrun every week and I also did a run of 30-45 minutes every week at 6:40-6:50. So in total I was running 60 minutes of more of "quality".
First result was 19:22 with a terrible last mile of 6:40. Next week I did 18:46 - a 30 second improvement. From there I thought I was going to keep improving until I got back to 17:50 but instead I plateaued and kept running around 18:40, with a best of 18:35.
I wish I was one of those runners who could go fast on not many miles but I guess not. I wasn't doing any intervals but I don't think it would have made any difference to my times.