I don't think you benefit much by going over 10 miles at marathon pace, especially if it's part of a 20 mile run. The risk begins to heavily outweigh the rewards. The body holds about 18 miles worth of caloric energy that it is ready to burn (give or take a few miles depending on numerous factors). So what you're telling me is that not only are you doing 10 fast miles, but you're doing them in circumstances where you might exhaust your body. I just don't see the benefit of doing that during your marathon buildup unless you have specifically set aside several days for potential recovery and some less intense workouts in the week or two to follow. Because that's precisely what you would need to do to allow for adequate recovery. And so I hope this highlights the point that if you just backoff slightly on a big workout that's pushing you to the edge, then what it typically allows is for much better training throughout the week where you never have to feel pushed to the edge.
I always felt I trained best for the marathon when I did a cycle that built up to these big workouts
8x1mile midweek at 10k-half marathon pace
brisk half marathon weekend at marathon effort
5x5k midweek at marathon effort
long run of 2-2.5 hours at easy effort
Now here's what I want to point out about this progression. On the alternating weekends where I did a brisk medium long run at marathon effort I always struggled. I would either have to keep it fairly laid back with a fast finish if I was going to hit 13 and if I went marathon effort from the start I frequently found myself stopping by 10 miles. It was just asking too much from a non-race situation. And it did make me nervous that I wasn't prepared to hit my goal.
What you have to realize is that you are not racing your goal race today, which means you should have no real expectation of being in goal shape today. You'll notice that I ran marathon effort, which is to say that I ran at a steady state that was aerobically fast, but never pushed threshold. The moment you push threshold at all you are done in the marathon. Can't do it unless you are a 2:05 marathoner in a 2:10 race. The rest of us crossing our fingers for an ideal race where we PR exactly at a difficult-to-us pace are inviting trouble if we think we can treat our marathon pace as something that ever gets remotely threshold in training and then expect to go out and run 26.2 at that pace on race day.
So what I'm saying to you is twofold. First, you need to start thinking in terms of marathon effort instead of marathon pace. Second, you need to back off in training, because marathon pace by definition should only really come together on race day when you are at the peak of all this conditioning you put in.
Ten brisk miles are fantastic. But remember that a half marathon at half marathon pace is actually an all-out race. So if you screw up your pacing or how you feel on a given day you might easily go into the red going at a race effort to do more than 10 miles at what you think is going to be your marathon pace. Don't risk it. Have more faith in yourself. I can tell you that 80-90MPW is more than what I did when I first broke 3 and I'm a guy who has always relied on mileage more than speed. It sounds to me like you have the engine to do this, but if you were to admit to me that these heroic workouts took you more into the red than they should have, well, then we can't make good predictions from them.
My suspicion is that you totally crush it. Don't back off by more than 10-15% in the final couple of weeks. Take the last three days before the race pretty easy, but do some strides after your easy few miles. If you can't sleep well the night before the race, then good, because that means your body benefitted from taking the last few days easy and you are ready to go.