The answer to that is part physical and part mental. Yes, there is the necessary endurance and stamina that comes from training and running many, many miles, but it's the mental part that I think really does it.
We train our bodies to be able to endure many miles and to run at a pace that we want to hit. But physically, things begin to happen, things begin to hurt and get tired. As we chug our way through the miles and as we stress muscles and bones, at least two things start to happen: 1) we start to feel "tired", and 2) something somewhere will start to hurt or ache.
And this is where mind over matter comes in. We start to feel fatigue as a result of the release of lactic acid from our muscles (which is a natural result of the energy release from muscle action). Lactic acid works as a basic signal to the brain to back off the level of exertion. In extreme cases, lactic acid build up will create a sense of nausea, which pretty much forces the body to ease up on the level of exercise intensity.
So the trick here is to recognize that muscles are releasing lactic acid, thus the indications of fatigue, but know that there is still muscle endurance left. It's almost like "ignore that flashing red warning light - full speed ahead!".... The fine line to consider is being able to recognize when you are truly red-lining your capability and it's time to back off. And where ever that point is, it is different and unique for everyone.
The same holds true with aches and pains that develop during a run. Which ache is normal and which one needs to be tended to RIGHT NOW? After running a bunch of miles, it's only natural for different parts of your body to start to complain. You can run through most of them, but you've got to be conscious that there are some that you shouldn't run through. And again, there is no one answer since everyone is unique.
For me, sharp pains are usually not good. Ditto knee discomfort. Knee discomfort at any level is usually a big DO NOT RUN sign for me. But general aches and soreness I can, and will, run through, and oddly enough, they often dissipate before my run is over - sometimes only to bite me in the ass later in the day or the day after... but that's a different story.
For me the key is to focus on whatever my goal is for that day's run. I won't pick a distance that I haven't been adequately or properly been training for; for example if my long runs have been 10-15 miles I won't suddenly decide to do 40 or 50 miles on a given day, but I might do 20... With that goal in mind, I just start grinding through the miles, and as I've probably mentioned in other posts, mile by mile they click off until I'm done. At some point during a long run, something in my body will start to complain and I'll do a kind of "how bad is it" analysis, maybe adjust my stride, slow down, or something, but I'll try to keep running. I used to hate stopping for any reason (hey, the clock is ticking... can't stop for nothing...) -like, I would put up with a loose shoestring or a pebble in my shoe for a long time before I'd force myself to stop to take care of it - really stupid of me... But I've grown up now (58) and I'll take the time to stop to take care of things so that I can have a better and more productive run.
But as the miles accumulate I find that I have to concentrate harder on maintaining pace and form. As I get tired, my posture can get sloppy, and as it gets sloppy, I'm not running as efficiently as I could. The better my posture, the less likely it is that I'll hurt something else. And this can take a lot of mental effort.
It's hard for me to remember exactly what is going on in my head, but I know that I'm trying hard not to think of how many more miles I have left, but rather focus on running smooth. Of course, I've also gone through runs where I'm thinking "5 more miles... "x" more minutes (depending on my pace that day)... if I run faster I'll be done sooner", which can make the end of the run very hard... To focus on running smooth, I have this mantra that I think I picked up from Arya in Game of Thrones: "light as a feather, quiet as a millpond"... and I try to translate that into my footfalls, as in "land softly, land gently"... and not let my feet just "plop" because I'm getting tired and/or my legs/feet hurt.
If I'm really getting uncomfortable, I'll start going through the routine of telling myself that the discomfort that I am currently feeling is temporary and expected, and that it will be over as soon as I cross my designated finish line.
But probably the mental thought that gets me to the end of my run when I'm getting tired and I'm hurting is the thought that I've not yet completed the goal that I set for myself that day. And only when I hit that mile number, whatever it may be for that day, do I feel like I can stop to lick my wounds.
I do find it interesting that we as humans can develop our muscles to do amazing things, but it takes that grey mush between our ears to bring it all together... It's pretty amazing!
