[Update] VO2 I just learned what that was and how it relates to my performance…a little late and probably obvious but…
You can’t run, or at least I can’t, a marathon at your VO2 Max…mine is as stated below…aah now it all makes sense. More on the VO2 calculation…
In my case 8:33 to 9:00 miles is in that MAx range and my ideal is btw 126-144bpm for the record I have been averaging

http://upload.wikimedia.org/wikipedia/commons/a/ad/Exercise_zones.png
As i have learned about myself…I can do almost anything I put my mind to…however there are smarter ways to approach shit…
My heart rate from the chicago marathon…note the first hour my HR was in the red zone over 160bpm for more than an hour…never never again will i ever force myself to burn up 70 mins of max heart rate running especially when the average temps are rising toward 82 degrees in the next 2 hours…I suffered terrible cramping and it turns out that my physiology had not trained over the last 9 mos this way so FAIL…did i finish a marathon YES did i enjoy it like i enjoyed my last 20 miler in september shit no…lesson…run as you train…i did not train this way nor will i succeed running this way…ever. So never again, live and learn.
the first 9-10 miles run overclocked my heart which effectively ruined the rest of the marathon….heart rate after heart rate recorded over the last year tells the story…next time i will listen to my heart rate very carefully and not force myself and have a good run.

How not to manage my heart rate...overclocked my heart rate in the first hour blew my whole marathon

http://mwelsh.com/e/2010-10-16_2004.png
By contrast the run below was ‘fun’ and i felt good after and during and still worked hard and temps at the end of the run were closer to the temps in the middle of the marathon.

This is my heart rate over 3.5 hours during my last long run 20
Tempo run thru valley forge park in the rain barefoot shoes.

This is a tempo run thru valley forge park avg hr 146bpm
8:41 min/mi for almost 8 miles below note the heart rate balance

This is a tempo run 7.28mile run at tempo speeds and i felt great at the end note the heart rates....effin idiot i am
This is a 14 miler a great run with hills and no heavy heart rate burn up front…

14 miler normal heart rate and plenty of elevation.