Trackman, with all due respect, you are having/had the same problem as the OP. You are obviously an FT runner who never quite got how to train for a marathon correctly.
There is a guy in my running group that sounds exactly like you. While my PRs are not great - as I didn't start running until I was 28 - this fella has slightly better PR's (15:30ish 5k) and has never broken 3 hours for the marathon. He has tried many times.
On numerous long runs, he would get frustrated with our slow paces and take off and run 6 min pace which was easier for him to run as his anaerobic system was quite strong.
If indeed you are as FT as I would guess, then you should NOT run a PR for the 5k/10k a couple of weeks before the marathon. The only leads me to believe you ran too fast during your training.
And don't get me wrong the Hadd/Lydiard method includes a lot of fast running and is as hard as any type of work you will find, it's knowing what type of workouts, and when to do them is what really matters.
Here is another Hadd comment:
"I like long easy distance to be M+75 to M+90 (M=marathon pace), this should require a HR roundabout 71-75% HRmax (assuming good training). Maybe even lower.
For training fat-burning pace (always assuming well-trained runners) is between M+20 to M+40 pace. BUT, this will (obviously) require a higher than easy HR (more like 80-85% HRmax). Training in this zone will improve the rate at which you can burn fat (convert fat to energy)
Obviously, these two training paces (70-75% and 80-85% HRmax) are doing different things. Once I know that someone can run 20-22 miles at long easy pace without falling apart, and do them regularly like it's no big deal, I usually (as you'll see on some threads) like them to begin to run the 80% or M+40 pace within (some of) their long runs. So you'll sometimes see 1hr45 easy including 60 mins at 80% HRmax. Although the following Sun you might see 2hr+ at easy HR again.
Putting the two together in one run (21 miles including some time at M+30 pace) is usually too tough / requires too much muscle fuel. Hell, just running 21 miles is often enough for most folks, without asking them to run some of it at the higher intensity (M+30). So, I'd say either run 21 miles at easy HR (75%±) (and do a 85% work session during the week as usual—not before Thurs after a 21 miler on the Sun to have time to refuel) or run shorter on Sun (say 2hrs) but include 60 mins straight at 80%± HRmax. One of the other, but not both."