I know someone who set a minute PR off 'easy' runs. But he ran higher mileage with consistent steady effort runs when he felt good. Training was like SoM (emphasis on higher mileage and easier tempos) but his fastest run would be like 5:10 pace to close out a long run. Ended up running low 14s. Mileage is so important
I'm a newb but I've learned quite a bit from Coaches Education sites. Old, antiquated layout but some of the most informative sites have had great running info.
Also, Central Park Track Club has helped. Another old site, but some of their links give similar advice about mileage and base building. Thank you CoachB!