Running is definitely mental.
When it's nice out I run a nature trail that goes through the woods, past some nice houses, and it's really nice. The trail ends at a busy road/parking lot.. And seeing that I park at the other end, it requires running to the lot, and back.
It's interesting.. I used to assume I needed to stop for a break (walk for a few) when I hit the mid-way point, but lately I've started to get to the lot and turn around and just keep running. Turns out, I didn't really need to walk at all, and could just jog a little slower pace.
Granted, a lot of this depends on how tired you are when you get going, but it's certainly very mental as well.
I like running a lot because it really kicks your ass, gets you fatigued, breathing heavy, and beat, and while that kind of sucks, it's also great because it's just a good feeling to know you are doing excersize that is fatiguing enough to do that. And later, after you can breathe normally again, it feels great!
I find the hardest part of running/working out, is getting there. After I get there (trail, gym, whatever), I find that I have no trouble "getting going." But it certainly is an effort to get off my ass and go sometimes. Realistically, I'd rather sit here and listen to Murphy Lee and browse the TFP, but ultimately, I know that running is going to be just as beneficial..
Also, one last comment on the mental thing.. I worked yesterday till about 9:00 PM, was kind of beat/bored/tired, etc. But then I came home, ate some supper, and decided to go running around the neighborhood at 11:00 PM. I had already run earlier in the day at the trail I mentioned above.. And I was actually surprised to find that I could run a good distance at a reasonable pace without dying.. Especially given the fact that at work, I was bored/tired. But clearly, a different kind of tired.
Anyway, that was a lot of words.. I'd say that the mental aspect has just as much (or even more) influence as the phyiscal, because in order to get to the physical part, you have to be mentally prepared for it. And while doing the phyiscal part, you can't let the mental part take over, or you will stop before you've truly reached your physical limitations.
Take care.