HR - The plan was 1:23 at halfway and a slight acceleration to sneak under 2:45. Just couldn't settle into the pace and got it a bit wrong so left myself with a lot to do. I think it relaxed me a bit as I thought 2:45 was probably gone - I was still showboating and getting the crowd going at Big Ben! It was only when I looked at the watch at 26 and saw 2:43:xx that I realised it was on. Still makes me really smile when I think if it now
That's not quite the full story though. By training over time you can teach your body to to use a higher percentage of fat vs glycogen at a given pace. I only took in about 200kcals but ran quicker than the McMillan calculator said I could based on HM and 10k times. My lactate threshold is 5:53/mile and I ran the marathon at 6:18/mile so it was pretty close
Also, people like Ron Hill (and many others) ran 2:09 in the 1970's without any fuel at all. Same goes for Steve Jones in the 80's.