tmbrntt... IMO, it's not worth spending 2-3 hours pfaffing about to get the 5k in on the actual course. You will lose way more in mental and physical energy than you will gain from 'knowing the finish'.
Further... Does this mean that one good in-race fuelling strategy could be to take on gels in the first 8 miles (giving time for glycogen to get into the body in a usable state for the last few miles???).... and then just take jelly babies towards the back end of the race to help encourage the body to release its emergency stores of glycogen?
Have you found a way to get round Newton's Laws of Motion?
RicF I've seen you make the 'stomach as a fuel tank' quip before but thankfully Gideon has been rather more helpful! I have read around the subject to some extent... but maybe I too have fallen into this trap a little, without knowing it.
Gideon... your explanation is really interesting thank you. Do you have a reference to a good book or article that covers this? I have read up a bit on the subject.. including the Pfitzinger and Douglas marathoning book, and I don't remember that explanation being given.
Your explanation would solve a problem I've had in understanding P&D. There, it tells about the benefits of training and increasing your body's ability to store glycogen in the liver and muscles. But I don't think it really says why... apart from the obvious that your body needs glycogen as fuel. But I've had this nagging feeling that my natural limit of about 18 miles is enough.... because I can just take gels during the race to get me over the line.
And why bother risking putting on a pound in weight by carb loading? Why train specifically to increase my glycogen capacity to say 20 miles. Surely the easy option is to take 8 milesworth of gels instead of 6. This is just one extra gel..
But your deeper explanation points to some real essential value in having glycogen pre-stored. Are you really saying that ALL (or say, 95%) of your glycogen that you use in a 3:30 marathon is already in your muscles/liver before you start? I imagine that pre-stored glycogen is crucial for a 2:20 marathoner, but much less important to a 5hr marathoner. I am perhaps somewhere in between. Any further insights from anyone would be interesting to read.
First of all, a 1:44 HM puts you in great shape for the a sub 4. I ran my first marathon in just over 4hrs off a 1:47 half.... and I've never run sub 1:44 but my subsequent 3 marathons have all been sub 3:50. I think you gain from experience, and a cumulation of endurance over 2 or 3 marathons... and you've said yourself, you've done more training this year. So I think you should go in with confidence. If you look at McMillan and other "race time predictors", I think they will all predict you can run sub-3:40 or sub 3:45... although many runners find them slightly optimistic in predicting marathon times.
Manchester pacers have a great solution for you... I just took this from their website.
"To help in your mission to hit a specific time, we have recruited a group of experienced pace runners who will be running the race at the following paces:
3:00, 3:15, 3:29 and 3:30, 3:45, 3:58, 3:59 and 4:00, 4:15, 4:30, 4:45, 5:00"
So if you're desperate simply to achieve 3:59:59... then if I was you, I would go with the 3:58 pacer.... and expect a relatively easy* race. (*there is no such thing as an easy 26mile race!)
But with that half... if your training has gone well, and you're prepared for a more gritty race, you have every right to start with the 3:45 pacer and perhaps accept that you could slip back a bit.... but I think you've a realistic chance of grinding out 3:49:59.... which looks even better (and don't forget, if the race time predictors are correct, then 3:45 would get you guaranteed non-sponsorship-based entry into London as a good for age entrant - if you changed your mind about this being your last marathon!)
Maybe I'm overplaying it... it depends how adventurous or risk-averse you want to be. Just make sure you read up on fueling strategies. too. And enjoy the challenge.