Exactly this. Your half marathon should have been practice for your marathon pace - so not all out. And no need to smash out 25 miles at that pace 3 weeks before a marathon. It takes weeks to recover from a marathon so why run one just before the big day?
I disagree about the Half. A Half 5 or 6 weeks out should be a test of your fitness. It should be raced. How are you to extrapolate that Half Marathon performance to gauge goal marathon race pace if you are sandbagging it?
Also, as I said earlier in thread, his 25 miler wasn't 20 seconds faster than GMP, it was 20 seconds slower than GMP, but that is still to much effort for a final long run; too long and too fast. He should have capped it at about 22 miles and run around 8:45 - 9:15 if he was aiming for 8:00 goal pace in the marathon.