All in your head

I read somewhere that running any distance beyond 3 miles is all in your head. I can only really run 3 miles, so I wonder if that means I have a weak mind. That's a depressing thought, but I wouldn't doubt it. How do I make my mind stronger? Jedi mind tricks? Wiggling my ears and rolling my eyes at the same time?
Or how about just flipping running more than 3 miles? Just DOING it. Somehow, I feel like there's some secret thing I need to do, some magic spell, some person I have to become and then POOF, I'll be able to run 10 miles. Telling myself that's not true doesn't seem to get the concept through my head though. Feeling guilty and weak just discourages me into not trying anymore, and that's no good.
The physical benefits of my quasi-running are obvious. My cardiovascular fitness has improved monumentally, for instance. When I started, I'd easily break 180 bpm running an 11 minute mile, now I'm at a comfortable 140-160 running a 9 minute mile. Can't keep up the 9 minute mile for more than 5 minutes, but hey.
Maybe the key is to just stop obsessing about 5 minutes and 9 minutes and 3 miles and just run all these silly thoughts out of my head.