12 miles is ouch! I have been doing 10-11 miles one day a week and when I stop it's like all the pain just settles into my muscles. OUCH.
Yea, a bit faster can be a lot more effort. I find this with swimming and cycling as well. If I swim a 1:40, 100 yards I'm cruising and relaxed. I swim a 1:36, 100 yards and I am working REALLY HARD. Something that I can't sustain.

