I would say now that Mike and I have been running for awhile that our typical run during the week is about 5-6 miles. I can bust out a 5 miler like it is nothing, most of the time, and not feel completely useless for the rest of the day.
The funny thing about running is how two statistically similar runs can feel so drastically different on your body.
Yesterday called for 5 miles, and since Mike and I were going out to dinner with our friends I needed to run before Mike got home. It was unseasonably warm yesterday, almost 80, and as soon as I started out on the run I felt thirsty. I didn't start to get really hot until the third mile or so, then I was just pushing to try to get the run done (and keep up a decent pace).
I am, usually, able to get myself through a run shorter than 10 miles by thinking to myself, "just get to the halfway point". Once I get to the halfway point I can finish up the run without any major issues (though sometimes my pace does get slower than I would like).
Today, I also had to run 5 miles. I started out a little bit earlier so it wasn't as warm but still not typical temps for October. While my pace was almost exactly the same as yesterday, I felt a lot better. My calves were really tight the entire time, but other than that I never felt as bad as I did yesterday. My paces were a lot more consistent today, and none of them were above 9 minute pace.
You would think that runs that were, in essence, exactly the same would FEEL the same. That isn't the case with running. Neither of these runs were the "I feel like I am flying and I never want to stop" kind of run, but I felt dramatically better today compared to yesterday.
That's one thing I love about running. Even though runs that I struggle during can be a little bit frustrating, they remind me that a better run is, literally, just a day away.