I have been running for a few years now, but have never really been able to make it past the 3 mile mark. I think it was somewhat physical, but after a while, it became mental, too.
When we were in California, we spent some time with Robby Pitt (the city strategist there), and he is a for real runner. I was telling him about my 3 mile block, and he suggested trying to run for 8 minutes and walk for 2, continuing with those intervals, to see if I could push myself past the 3 mile mark. I finally tried it earlier this month, and have gradually increased my distance until, finally today, I hit this!
I don't always have an hour to run, so I won't always be able to go this far, but it's super exciting to me that I can sometimes.