There is always a pace that you can run at that will prevent you hitting the wall.
I would say that your weekly mileage in the build up is going to make 8 mins/mile a little too quick if you want to avoid a very uncomfortable last 10K.
How did you feel towards the end of your 22 miler and what pace was that done at? Presumably, it was at something like 8:30-9:00 mins/mile?
Your optimal pacing strategy may well be towards the top end of that range, any quicker and you probably risk losing time hand over fist towards the end.
The half pace is not too relevant in these circumstances unfortunately.
If you had been averaging 70-80 miles per week over that period then, yes, you could look at aiming for a time related to your half pace and probably be easily under 8:00/mile.
However, to do that you need a really well developed capillary network to keep pumping loads and loads of oxygen to the muscles. Fat uses huge amounts of oxygen when being metabollised. The only way to get the required capillary development is through piling up the steady miles and that is the area in which you have struggled.
With a restricted oxygen delivery there will be propotionately more glycogen in the fuel mix (which requires significantly less oxygen to metabolise) and obviously the quicker this is used up the sooner you struggle.
So the aim is to go slow enough to deliver oxygen through the limited capillary network at a rate that fat can be burnt at.
I agree that on the face of it 8:20 looks slow compared to your 7:26 half pace and I'm sure it will feel easy during the first half. However, you could still be using glycogen too quickly to make it the optimal pace.
Hope that makes some sort of sense?
Thats a tricky one and was why I was asking about the pace of your 22 miler.
There was no obvious dropping off of pace during that run (I assume the variations were as much to do with changes in terrain as anything else?) and you felt as though you could have continued to full marathon distance at that pace.
So that basically gives you a line in the sand. The optimal pace is certainly no slower than 8:53.
At the other end, if you ran your theoretical marathon pace of approx HMP + 20secs/mile, ie 7:46/mile, I would estimate from your weekly mileages that you would crash and burn around 18 miles. At this point you could expect to lose anything from 1-2 minutes/mile for the rest of the journey, ie 8-16 minutes.
Now if you did slow down by 2 minutes/mile (and believe me that is not at all stretching the point) then you may as well have run a full 1 minute/mile slower in the early stages and kept an even pace throughout (and enjoyed the experience far more into the bargain).
So, optimal pace is a good deal slower than 7:46 and quicker than 8:53, where exactly in that range is a bit of guesswork but I have a hunch that it will be 8:30-8:40. You may just about get away with 8:20 but just be cautious.
Whatever you manage though, even if you're on your last legs at the end, you'll know you can go much much quicker in the future just by getting more steady miles in.
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