Like It Or Not, Your Whole Body Is Connected
When I started getting my weekly running miles up and hitting 8-12 milers with more regularity, I started to notice different ways that my body was responding to the stress. You expect to get fatigue in your legs, sure, and that has certainly happened, but I've found it has been happening less often as I get more adapted to my training regimens.
I've read about how weight training for your whole body is beneficial to running so that's something I knew intellectially, but I didn't know it experientially until I started to feel muscle fatigue in parts of my body besides my legs while running. The first place I felt that was in my core. That wasn't so surprising because I can definitely see the connection of the core muscles to the parts of your body engaged in running and core training can help with endurance among the myriad other reasons that you can find to support a correlation between core training and running effectively.
More recently, I have started to feel muscle fatigue in my biceps and triceps. Those were pretty surprising because it never occurred to me that your arms are really engaged when you run. This first happened on a 10 or 12 miler so I had ample time to think about the mechanics of what was happening while I was running. Even without any weight, you arms are helping to support your momentum and are going through repeated motions that don't require as much energy as your legs do, but they still do require some energy and endurance.
The more you learn about anything, the more you see that so much is connected, whether it's history, anatomy, literature, or just about anything else. The more I get into something like running, as well, the more I understand concepts like upper body strengthening as being a benefit to running, particularly when you start getting your miles up.
So, whether or not you want to believe it, just about everything that happens in your body is connected.