I was able to get to 4:50 by just doing 25-30 mpw. But when I stayed with the same amt of mileage, my mile time stagnated / only improved 2 seconds in the course of 2 months.
It took me 40-50 mpw for half a year in order to drop to 4:28, and I doubt I'll ever get below a 4:20.
That being said, what about you guys? We all have this threshold wherein once you pass it (like the 18 min 5k barrier), it becomes a lot harder to improve.