I've just come across this thread, and haven't read all the comments. However, can i suggest you get a book: "Adrenal Fatigue", by James L. Wilson (and/or do a google)?
You may find (as I have) that your adrenal glands have had enough for a while and cannot produce all the chemicals your body needs - and then the thyroid and other bits get starved - hence the feeling 'beyond tiredness" that you may experience.
I'm a regular runner who has averaged 40 miles/wk for around 20 years - I'm still getting faster, but this year I've been hit hard and couldn't explain it. I now know the difference between 'just' very tired and exhausted, and fatgue. Several other symptoms and you have to stop and take notice - believe me. You can overcome it, but it may not simply be the thyroid - that is likely to be a secondary effect.
and your PB sounds really good. It is not the easiest of courses, so you'll have no problem in taking chunks off that in due course, on a flat course.
Unfortunately, I will not be doing Chelmsford - or any for some time. Whilst I seem to have stabilised things, my running is going to be very gentle and sporadic, until I've got everything else 'back in the box'. I've got so much on at work, etc. currently, that I've had that sign that I cannot continue to do everything and avoid sleep, etc.
Don't want to lose touch though and I know I will be doing PBs in 10 months time..........