I agree with otter. I think this post is right on. I'm probably not as good a runner, but I think I've figured out why I've fallen apart sometimes and raced well others. Let's say 2 different runners are doing the same workout am same interval length, pace, recovery, everything. One did more miles first this one is getting a different stimulus because he's starting with less glycogen and more tired.
I have a workout I've done for a half marathon. There's a place I can park with both a paved bike trail and a track. I'll do a 5-1/2 mile tempo then some track work - 300s usually. The course allows the tempo to be 6-1/2 also and one can also do quarters. This is actually similar to a special block workout for a shorter race like 10k. Wetmore had Goucher do a morning tempo then short repeats in the afternoon. This is just one session, not a daily double, but the same 2 stimuli.
So I've done it in the past and now I'm training for a full. I went 6-1/2 first, then did the half workout, maybe the longer version if I remember correctly. The whole thing was between 18 and 19 total including short cool-down . Big difference in overall distance, and the fastest running was late , after a half marathon of running already. It's not super marathon specific, and if this is your one big key workout you're likely to fall apart in the race, but it was early in the program.
Later on I did do some very specific work with a half marathon or more of hard running including several miles after the 20 mark. The earlier workout allowed that. You have to be able to manage marathon pace after 20 miles of running. It indeed DOES matter how much you run before the hard part. A workout with some faster than race pace running in the 22-25 mile range of a longer run would be ideal in my mind and, I believe, the poster I quoted would agree.