I trained under the guidance of JK and experimented much. Nothing is proven but here is my experience:
I did mileage where I averaged 140s, 130s, 120s, all the way down to 20s.
120+ is unnecessary and I'm sure many of you agree.
The absolute most important factor is threshold runs; the high end runs and tempos. If you can master these than you can be very successful from just 50-60 miles a week.
I would say 90-105 is ideal with the mastering of the threshold. nothing wrong with reaching 120 even if its for the mental reason: "I ran 120 miles per week. I cannot blame my mileage as the excuse for my poor training / performance."
When it comes to end of the season, I had great success even at 25 miles a week. As long as you hit your workouts then your training will improve you. You can, in theory, take your easy days off or I would recommend light cross training. every workout: "what is the purpose of my workout today? How will this effort make me better?"
never be afraid to cut a workout early if you feel the workout will not make you better (maybe because you feel like crap or , simply, you're to tired.)