The only time I believe I really suffered from fatigue, (it might have been something else?) was in 2015 in the Pyrenees. There was one day when I just pushed myself too far, I hiked from dawn until dusk. By the time I arrived in St. John Pied de Port I could hardly stand up, my eyes wouldn't focus, my speech was slurred, I was shaking from head to toe and I felt really cold. It was a very hot day! I recovered fairly quickly but I have never been that stupid again.
Had the same problem myself on the Cleveland Way after a hard 33 mile day.It was July and very hot yet I was shivering in the tent despite having warm drinks.It took a few hours to recover but it was very unpleasant.
Found the guy's blog.http://stevebirkinshaw-wainwright2014.blogspot.co.uk/search?updated-max=2016-02-22T06:46:00-08:00&max-results=7
Quote:"My symptoms seemed to have settled down, instead of just complaining about feeling tired I feel I can now catalogue them.
1) Persistent exhaustion that has now been going on for 6 months. But there were also signs ever since I completed my Wainwrights run. So even 12 months ago after a long hard race it would take me two weeks to recover and feel up to running again.
2) ‘Brain fog’. I cannot think straight or concentrate and my short-term memory is absolutely awful. This normally clears around midday and by the evening I feel back to normal. But on a good day it might clear at 10am, on a bad day it might never clear. When the ‘brain fog’ is bad I find it impossible to write a simple email or even remember a simple instruction.
3) The persistent wish to go to sleep even when I have just had an 8-9 hour sleep.
4) Disturbed sleep. If I am in a bad phase, I wake up in the middle of the night and cannot get back to sleep
5) Regular headaches.
I also now know three things that make it worse:
1) A long fast fell run (90 minutes or more). Particular bad is a long hard run before breakfast. After this I can feel shattered for a week or more.
2) A big drinking session. I have just had my works Christmas ‘do’. For three days after it I was exhausted and I found thinking and concentrating nearly impossible. All I wanted to do was lie in bed.
3) A stressful situation, such as having to give a talk in front of a group of people. On the next day I will again by really tired and find even a slow run impossible.
What does not seem to cause any problems is a run (or jog) up to an hour, as long as I keep my heart rate down below 130 beats per minute. In fact being outside relaxing my mind seems to help."