EDIT: I am conditioned now to run 10 miles of hills in triple digits and not need to drink. I couldn't do that last year. I'm learning more..how to nose breathe and calm/slow my body rate down through breathing exercises...as I run. To concentrate on breathing is to be present. Try it...you will see what I mean. Concentrate on breathing.
for sure, focused breathing is the key especially to will mind over matter... you can always push just a little bit further... then a little further etc.
my best routine was 6 miles in about 45 min then 1,000 push ups and 1,000 crunches every night... I was 155 lb.'s and could bench over 300 lb.'s.
now I would be lucky to do a single mile below 8 minutes... I could do it, but would pay!