I've attempted a full 20 at MP four times in training. I nailed it all four times, and it felt fine. Two of the following races were big PRs, and two were mediocre.
And I know I've said this before, but I'm just going to keep repeating it: I'm not saying this is the only way to go, and I actually don't do long runs that are quite that hard anymore. I think there was a sweet spot in my early 30s when I had enough miles in my legs to really handle it, but age wasn't catching up with me as much as it is now.
This to me is the key point about these big workouts. In my experience, coaching myself, coaching others, and just talking to runners, it is very rare for amateur runners to hit marathon times that are "equivalent" to their shorter distance PRs. If there's a common thread to the ones who do manage it, it's usually that they do higher mileage and harder long runs. If you're a sub-15 5k runner and you're struggling to break 2:30 in the marathon, you are drastically underperforming.