Thanks for that. Warrants some really careful perusal (although I appreciate the summary).
Interestingly, a trend that you have mentioned here (and one that Tinman has also mentioned) that really fits me is that I could always get fit really fast, would tend to plateau, and then to "train off" and completely lose form. I'd sometimes be better after two weeks active rest than I would for the competition I was aiming to peak for.
I had one particularly marked instance when I was 56 getting ready for the indoor USTF Masters. About three weeks out I did 4x1000m with 3 min recovery in 3:19, 3:19, 3:20, 3:12, easing down on the last one. I really felt that on that day I was close to 10:00 3000m shape. By the time I got to the race, I'd begun to feel like I was going off, even though I'd taper. Felt crappy in the race, although I was fortunate that it was a rough, pushing, shoving, tactical race, and managed to get third in 10:22.
I've had injuries on and off since then, and never got back to a shape to make it worth going to a major meet, but I'd decided if I ever got to a point in a cycle where I was running super-well, I'd just go back to easy running.
I think for me it's part nature, part nuture. I grew up, and spent about the first 20 years or so of my running life doing three track session a week, or two and race, often of very intense nature. In England in those days in winter on club nights you would twice a week for 'steady' running, you'd often run between 7 to 10 times at well under 6:00 min miling (and usually get dropped).
I also discovered via dna testing that I'm really a slow sprinter with a big V02 max, and fast lactate clearance. It means that reps/intervals really tend to flatter me because I'm comfortable with the speed (or was), and even though I might have deeper oxygen debt than some at the end of the rep, I would clear lactate quickly, and be ready go again off a fairly short rest. Even a few years back on a McMillan plan I was running 24x200m with 200m jog at 1500m pace, until I discovered they were supposed to be 5k pace. Because it's relative easy for me compared with long runs, I always tend to gravitate to that kind of effort - I must be one of the rare people who'd rather do 20x400m than an easy run on a scenic trail.
I had McMillan coaching plans for about three years fro 53 to 56 until the injury cascade began. After I'd got myself into an intensity dead end in my early 50s (I thought after 35 years of running, I need something MORE intense to get adaptions), I'd realized I need some different input.
It did work well, although I probably ran a lot of the sessions too quickly, and they did get me down to 17:30 for 5k in my mid-50s, and more surprising 37:03 for 10k on a fairly stiff road course, which was about 87% age graded, a huge shock given my generally mediocre endurance ability.
Anyway, I'm hoping that another 4 or 5 weeks, I can actually start training systematically again. I'll have to try some of the longer runs on the trails to slow me down. On the roads, I just seem to hit a comfortable pace that's way too quick for polarized training. The last 'long' run I did before I got injured was 8 miles at 7:22 per mile, so if I'm on the roads it's going to take a conscious effort to hold back.