I've never been of the "I can't do it" mentality; essentially nothing is impossible in my book. For someone who aspires to run all day, and longer, this mindset is a great asset, but it's also a double edged sword.
The ability to push on when you're tired, when you've been running/walking for so long that your body makes you sleep whilst you're still moving is something that I think we all have. Although some have to dig quite deep to find the mental reserve that enables you to separate the mind from the body so that this weak thing that we're attached to can be pushed through the tiredness and sleep barrier. To keep going when sleep is the only thing you crave, "Just 5 minutes, I'll be fine after that". To push on when the tears flow because you're just so exhausted; these are just mental battles which make me chuckle when I think about past times that I've been there.
But what do you do when your body simply can't do what you're asking? When pushing on breaks your body?
Most of us push on at least a little, because it's that little push that makes us fitter and stronger. Personally I find it hard to gauge that "little push" which is why I'm prone to injury; and why I'm sat here (again) with a lower limb strain/injury/thing that stops me running/whatever.
This never used to happen, but ever since I turned 40 a few years back it has... So it would appear that I can no longer smash through the tiredness and aches and pains to emerge on the other side stronger than ever. I have to make slower progress now, and really abide by the 10% rule.
But with a wry smile I still consider that as I get fitter, and stronger, maybe I will be able to go back to the ways of my younger days *eyeroll*