For me, distance running is mostly about pain management. It's fun for a while, then it starts to hurt, and then it's just a matter of how much pain I'm willing to put up with to reach a certain distance. This has taught me a few important lessons.
Particularly because I run barefoot, one very important lesson is knowing the difference between pain and damage. Running on gravel hurts more than anything but it doesn't cause damage. Getting a small piece of glass in your foot doesn't hurt as much immediately, but requires attention. I'm pretty good at telling these apart now. Similarly, most aches and pains in my legs amount to niggles that go away if I keep running, but once or twice I've needed rest to heal ... I don't have enough experience to tell those apart yet. Meta-lesson: pain sucks, but a lot of the time it can safely be ignored.
Another thing I learned is that, at the end of a more extreme run, pain comes in waves. One moment I'm really hurting and wanting to give up, but then that passes and I feel a lot better. Then that passes and I'm feeling terrible again. That cycle repeats with a period of perhaps a few minutes. Knowing that it comes in waves helps me not give up in those troughs ... another meta-lesson.