My two cents: If we eat well before the race with a good carbo load thursday, friday and saturday , we will have about 500 gr
of glycogen stored in our muscles and liver .Now 1gr of glycogen give us 4 kcal , so 500gr give us 2000 kcal.
Running cost is about body weight x km. So for example 10k race 60kg x10 = 600kcal (no problem with the fuel
is not even a good thing going into the race with 500gr of glycogen in our muscles and liver cos glicogen keep also water in our body so we can have a kilo or more of weight to carry along in the race.
21km 60kg x21 =1200kcal (also here no problem)
42km 60kgx42= 2520kcal Here only the glycogen dont let us reach the end of the race at our given pace we need energy
from fat .
But let's look to an elite runner like Eliud Kipchoge 1,67 x 52 kg
42k 52 x 42= 2184 kcal a runner like Kipchoge can run a full marathon almost using only glycogen with a little help from fats. And using only glycogen he can run far as about 39km!!!!
But if we take an other runner probably not elite of 70kg
42k 70 x 42= 2940 kcal
Using only glycogen he can run far "only" 28 or 29 km . So he needs a lot of help from fats to complete the marathon.And we know
that this is not easy.
I want to share one anecdote from my training group-club. There are three guys with pb of about 35' and 36' in 10k
and 1:18 in the half and since three years they try the marathon and they are fighting to stay under 3 hours .
Their body weight is about 70 kg
There are also three girls with pb of 38' in 10k and 1:23 in the half and at their first marathon made 2:52, 2:54. 2:56
Their body weight is about 50kg.
Same coach and same training.
There are not to much secret about it, they just need less calories to complete the Marathon and so they need little help from fats to complete the race.
While in race where fuel is not a problem the things are different.