Way to go on the fantastic run!
I do the same thing as you, it sounds like. If I'm running outside I'll set a visual goal when I'm feeling oogy but if I'm feeling good when I get there I'll set another goal for further down the road. On the treadmill, I'll sometimes say "OK, I can make it two more minutes to give a nice even number" and then when I'll get there if I'm .2 of a mile from completing another mile I'll push myself to finish that mile.
I also do so much better when I can turn my brain off a bit. If I'm paying too close of attention to my time or my distance I focus too much on how I'm feeling and I tend to take breaks more often. If I can get focused on the TV on the treadmill or looking around me outside I find it easier to just keep motoring on.
Wife to one, mama to three, wishing there were more hours in the day!