Interesting - I put my paces on an armband so at each mile I know where I should be time wise.I usually work these out from training runs and my desired target time (knowing that HR will be a little higher on race day). But then I'm only doing halves so probably easier to cope with some increased HR.
Clearly the last section compared to Abingdon showed something amiss - HR dropped along with pace so my guess is with the fuel you mentioned you would have been OK. C'est la vie - at least you did it!! There are literally millions that couldn't EVER!
Nathan - you are right, recovery usually takes longer than you think. Lots of different variables unfortunately so no given time. Some people do a reverse taper over say 3 weeks but to be honest it could take longer. I wouldn't even think about a 2400 test until your stats are back to pre marathon pace / HR and stable for about 3 weeks.