My experience was that I could run quite hard on hard days as long as I didn't also succumb to the temptation to run hard on easy days. It would frequently take me 2 easy days to recover from a really tough hill, interval or fartlek session and when I was running smart, I took both those days in between quite easy. Other times, I would just take a single day and then hit it again, and that turned out to be far less effective for me. The most effective recovery days were something like 3-4 a.m/6-8p.m. As long as I kept those recovery runs at slower than 6:30, I was golden. In hindsight, easy days could have been even slower, say 7:00.
The hard days were often sufficiently hard that I was unable to jog my cool down and would have to walk it instead. For example, 3x3 mile hill loop, working hard up the hills and gliding the flats. These were untimed, but done with each loop progressively harder and zero recovery between loops, basically 9 miles of progressively faster hill running. 4x2miles on the track was another nutcracker, running them progressively faster from 9:20-9:08. The slower ones would have been a bit slower than 10K pace, the last rep at or a bit faster than goal pace. Uphill on dirt track of course ;)
Another key for me was to eat my way to recovery. Thank god for the preponderance of free-food happy hours in the 1980's. A couple of Leinenkugels and all-you-can-eat taco bar right after a tough work out was a staple. I think there are studies out now confirming the "eat your way to recovery" ideas. I don't have to read them though - all I need to know is it worked.
Hard/easy/easy applied during aerobic base periods as well, though less pronounced (16/10/12 would have been a common 3 days). I never ran the same distance or effort every day. Year round I had good days and bad days.
PB's: 13:43/28:30.