Depending on how much and how often you run, your glycogen stores may not be at 1500 or 2000 available calories. Over Friday and Saturday I have 25 to 30 miles planned. If I do 18 on Friday there is no way my glycogen stores are going to be at full capacity for a 10 mile run ran either later in the day, or on Saturday if I have to do it then. Even for the 18 I have planned for Friday. I'm doing it in the morning I'm on a 10 or so hour fast before breakfast, then I go and burn 1,800 calories in a few hours. My glycogen is low from the night fast and I ate a light breakfast since I'm going for a run. So I fuel up during. IF you are not training to the best of your ability, your strength come race day will not be to it's full potential.
Depending on how much and how often you run, your glycogen stores may not be at 1500 or 2000 available calories. Over Friday and Saturday I have 25 to 30 miles planned. If I do 18 on Friday there is no way my glycogen stores are going to be at full capacity for a 10 mile run ran either later in the day, or on Saturday if I have to do it then. Even for the 18 I have planned for Friday. I'm doing it in the morning I'm on a 10 or so hour fast before breakfast, then I go and burn 1,800 calories in a few hours. My glycogen is low from the night fast and I ate a light breakfast since I'm going for a run. So I fuel up during.
IF you are not training to the best of your ability, your strength come race day will not be to it's full potential.
I did do doubles before but certainly not 18+10 miles. these were 2x1 hours at a pretty easy pace. still, I'll tell you my experiences, first run of such doubles was on empty stomach (often 12+ hours fast, I sleep a lot, haha + I usually don't go to bed right after dinner) and no problem. second run of the double was 3 hours after meal and it would go just like the first one, same HR/pace, same subjective feelings. so I think it was okay.
I actually found it pretty hard to empty the glycogen stores - when I deliberately attempted once I failed to do so. probably because the pace I attempted to do it at was too slow.
I have no doubt though that I would run out of glycogen if I tried to run a marathon too fast.
PS: I agree with most of what jimmyb said.