Hmm. I know you said mental exhaustion, but what do you do as far as eating or drinking during runs? I bring water with me on runs 6 miles or longer (drink every 2 miles) and bring fuel on runs 7 miles or longer (and take fuel every 4 miles...might seem like overkill to some but it works for me).
nitely not alone though. Distance running is VERY mentally challenging. Good luck!
That doesn't sound like overkill to me at all, especially when you're really starting out.
My body in particular is very sensitive to electrolyte loss (I sweat a LOT), so even on the treadmill, even starting when I was doing 3 miles, I started experimenting with electrolyte replacements. I'm still experimenting, but I'm narrowing it down to what really works for me.
I'm looking at it in terms of replacing salts, etc, in terms of taste, in terms of being able to eat/drink it while jogging or in a walk interval, and in terms of what doesn't mess up my sensitive stomach!
It's actually pretty amazing, how you can be really tired, then eat a few "sports beans" (the ones I've found are from Jelly Belly and I got them from REI but other runners say they are everywhere), and a minute later you just get a nice rush. It perks you right up.
The other day I filled my water bottle with water combined with a packet of something called Coco Hydro, which is dried coconut water, which apparently can be a good replacement drink. It tasted pretty bad, but I felt GREAT at the end of 6 miles. And that was on the "dreadmill", LOL.
I remember running in the late 80s and early 90s, when there was really nothing like this (sure, Gatorade, but I rarely drank that, and NEVER thought to drink it while training), and those runs were such slogs. I was always just wiped out, and I was a late teen/early 20s person back then! Now I'm 43 and with the help of these electrolyte replacement drinks and gus/gels/beans/etc, I'm feeling better than I did before.
That's the physical side of things, of course.
Good luck!