I find that there is a point during a long run when I realize that I'm "tired", and yet it feels good, so good, to keep running. There is no consideration of stopping. It just suddenly occurs to me that I'm a little tired. But it's a "good" tired. It's a "I can go for a long time" tired.
It doesn't always hit me at the same distance; sometimes it happens around mile 4, mile 14, or mile 18. It's a different feeling from a genuine fatigued feeling that is the result of not having gone through good endurance training.
That "good tired" is maybe the point at which I'm fully loosened up, I'm in the groove, and I'm just running... It's an automatic flow and rhythm. Things are in sync.
When I was doing my long, long runs during my marathon training, I remember that starting around mile 18 I would start thinking "don't stop running, don't stop running". My legs might have been grateful for a stop in the action, but since that wasn't in the plan for the day, I wouldn't stop. And that is definitely a different kind of tired. I know that I was beginning to feel fatigue. But of course the purpose of the training was to build up the endurance to run the distance without getting fatigued.
When I ran the Chicago Marathon, I don't remember getting tired. And I chalk that up to the training and running the arduous miles prior to the event. I remember being somewhat weary, like hey, I've been pounding the pavement for a long time, are we done yet? But perhaps the difference was that I had already pre-decided that I would stop running AFTER I crossed the finish line. Motivation and persistence are strong movers, but the training was key.
But I love that tired feeling, or whatever it is, that I get during a run. It's relieving, it's refreshing, it's re-infusing.