I have something to add to this conversation. I went back to my training log to confirm this information.
A couple of years ago, for over a period of three months, I decided to increase my mileage drastically. At this time I had been regularly running 80-85 miles a week for probably 5+ years and decided to see what I could handle. I have never been a slave to workouts and on many occasions, when I haven't been feeling it, I would bag a workout and just run longer. I increased my mileage to the 125-130 miles range for 3 months in the winter. It was a cold winter (avg of -5F) with lots of snowy/ice covered roads and trails. I essentially ran 40-45 minutes in the morning and 80-90 in the evenings. I tripled once. I'd run 3-4 accelerations of 15-20 seconds at the end of runs a couple of times a week. I wanted to do this more often but, to be honest, I could rarely muster the courage. Once a week I would run anywhere from 2h to 2h20. The bulk of this running was at 7-7:30 mile pace. In total, during this period, I counted that I ran 12 defined "workouts".
Here they are:
1) 10 x 60 seconds hills jog back rest
2) 25 minutes tempo
3) 10 x 60/60
4) 14 x 50 seconds hills
5) 2x 600m + 2x400m (long rest 3-4 minutes)
7) 6 x 1km (3:02-3:03-3:02-3:02-3:04-3:01) off 60 seconds rest
8) 400m - 400m - 800m (long rest 3-4 minutes)
9) 20 minute tempo
10) 6 x 4 minute tempo 90 seconds rest
11) 3 x 10min (3:15 avg) 90 seconds rest
12) 40 minute tempo
These weren't part of any program and they weren't spaced out equally, say, one a week, The first four were run in a ten day period and then I didn't run a workout for 4 weeks. I basically jumped in on workouts with my friends. I was surprised when I ran the 6 x 1km as it felt like I could run another 6 at the same pace and I hadn't done any structured workout in the previous four weeks. The last three were done at the end of the cycle as I decided to race a half marathon and figured I should do something specific to prepare.
I remember very clearly that I felt pretty tired most of the time (just a tad more than a recent period that I took a hiatus from running). I also remember that as it progressed I would occasionally "run to the barn" as Malmo would say. This was surprising for me as I was always a slow plodder type runner and rarely intentionally picked it up on runs. These quicker paces just came naturally and I didn't push it but had to hold myself back sometimes as I would really start to get after it. I remember, quite clearly, 5-6 times that I felt like I was in the best shape of my life.
After this, I ended up running 1:08 for the half, which was a pb of over 2 minutes. It was significant because my previous pb had been from a particularly hard bout of training (consistent 80-85 mile weeks in doubles with 2-3 workouts and a long run of 90 minutes).