As a relatively new distance runner (I was a sprinter in my school days), I found that I would PR all over in my build ups for marathons. I think this had to do with sticking to a plan (cause I can't get away with scraping through a marathon), running lots of base miles, and not stressing about shorter races.
I think the lack of substantial aerobic training is the big reason that training for shorter races just doesn't work as well. Lydiard had it right, everyone needs to build that aerobic engine base, then focus on specific needs after you're appropriate baked.
Tapers need to back off just enough activity to freshen up from the load of training, not completely stop running. I know I have hit it hard in a cycle after a big PR tune up race just to overcook myself big time.