In reply to Jimbo C:
> Maybe I haven't done enough long routes to comment, but the opposite has been the case for me; feeling good at first then feeling completely empty later.
Absolutely, if you're suitably trained and rested for an 80km run then the first half should be a doddle, unless you're running too fast. But the OP did say it was more of a mental issue than a physical one.
I always find the dip in the middle, but then tend to recover. My last big one was the UTMB - after the first 90km I was tired, but OK although I had a painful knee and thought it might be the end of the race. The next 20km I felt like I'd forgotten how to run and it was purgatory, then for some reason the rest of the race I felt like I was running on fresh legs.
Knowing that something like the above will happen, and is normal makes it easier to manage the hard times on an ultra. You know that ups and down will happen and that the race is long enough for things to change positively again.
When things are bad I tend to try not to disassociate the discomfort as some have suggested. Instead I start a long mental checklist, going methodically through everything in my head. It has the benefit that you start to put it in context - yes I feel sick but I'm going to change my food or lay off for a bit - or walk for a while. Yes I think I'm developing a blister so I'm going to change socks and check my feet at the next checkpoint etc. You can also go through the positives and I think it means you get a more objective view of your situation rather than just 'feeling' good or bad.
Of course you have to do what works for you.