Hey HL - I'm a week behind you and know that we are at a similar level. I think the whole building the marathon up in your head to be everything is part of the issue. It takes on such mahoosive proportions of mental, physical and social time that it all gets a bit off kilter. Not helped in some ways by the forums - don't get me wrong - they're great - but you can end up over thinking/planning/worrying/what iffing to the nth degree if you're not careful.
I really, really, really want to get sub 5 and I know I'll be disappointed if I don't. (even if I do have plan A,B,C and disaster too) But - and this is where I think you and I need to get our heads to - it's just a long run. We did Ashby in a real good time and 'enjoyed' it. (I use the word enjoyed in it's loosest form) The marathon is 'just another long run' - with slightly larger crowds. If it goes well - wa hoooo and I will feel really up. If it goes badly - it was just one bad long run in hundreds of long runs. Running isn't all about 'the marathon' and the marathon isn't the end, it's just one point along a journey.
I've tried lots of things in life that haven't worked out - some just aren't for me, some I might try a few times and then reject, others I might persevere at because they're doing something positive for me. It doesn't mean I've failed - that's a really harsh word - the experience just didn't live up to my expectations! You have to decide if the marathon 'does something for you'. If it does, great, once you've gotten over the disappointment give it another go, if it doesn't, move forward and start the next leg of your running journey - whether it's for fun, fitness, mental health, races or competition- it only has value if you value it. Running does not have to mean marathon.
The marathon has been your focus for the last 4 months or so, so it's not surprising you're feeling down. Find something else - not running related - that you can focus on and look forward to and watch your toenails grow back for the next couple of weeks!
Really hope you can see the positives in all of this, because there are many.