Hi steepler!
I have done a lot of track racing. Road, cross-country, hill/fell running...
And, for example, those times around 1h17' were all in different conditions.
In one I had the proper training for long distance. The next I was feeling good going for 1h15' and got injured in the last 300m!!! I had to stop and then walk.

The third, I was back from 3 months stopped and had trained for only 3 weeks. It just happened that day...
I think that some time ago the problem was psychological as I was afraid of not being able to run at PB pace. After that, like last Sunday, something unexpected happened (together with the lack of training miles).
It's a bit embarassing to say this but after the 3km marker and until the 18K one I was just worrying where to go... err... hmmm... to the "toilet"
After the 11K it got really bad and I couldn't think of anything else...
First time this happened to me in 10 years...