My usual morning run is 8 - 10kms or 5 - 6.2 miles. I normally get around that course in 40 to 50 minutes.
I found two ways to get through my mental block (I had one as well at about 3kms lol). One was to listen to music as I ran as this let my mind concentrate on something other than my body and it really helped me zone out and just day-dream really.
The second was running with another person...in this case my boyfriend (who happens to be 6'6 and i'm only 5'6...so you can imagine how many more strides my poor little legs have to do to match his lol). I am a naturally competive person and no way would I let him think I wimped out by walking :P so I ran the whole way...one i found i could do it once i've been fine with doing it since.
I've never run a marathon. I ran in the City to Surf which is about 8.6 miles or 14kms last year and will hopefully be running it again this year but thats about it.