Anything is possible. Well, except for the stuff that isn't like square circles and pro sports teams from Cleveland winning world championships. But I have a hard time believeing that someone is at their max on 45 mpw. I'm not saying it isn't possible and probably there are people here and there who can't do more, but they're few and far between.
That doesn't answer your question, but it may be that you were running too hard even at 7:20. In the glory days of the Florida Track Club the AM run was very slow, 7-8 minute pace and the guys doing it were pretty much all capable of running under 30 for 10km. So it could be that you just need to run for time and not think about how far you're going. eventually you'll get fitter and the pace will come down some. I'd also recommend vitamins if you aren't already taking them.
20 miles once a week in a 50-60 mile week leaves you with 5 to 6.5 miles that you're running on the other six days. I think you've got too many miles concentrated in one day and you're not getting in enough running nearly all the time to get through a marathon well. It's probably ok, maybe even good, to do a twenty miler or two as the marathon approaches, but I think doing that each week is putting too much strain into one day and not enough into the others. I don't know how fast you'e doing these runs, but I'd keep that long run to something in the fifteen mile/two hour range and add those orphaned five miles to a midweek run. So you'd have a bunch of fives and sizes sandwiched between a fifteen and an eleven (or so.) I'd also suggest that the majority of those 50-60 mile weeks be at the 60 end of the scale and it wouldn't hurt to add another 4-5 miles onto yet another midweek run. This is ignoring other aspects of marathon prep.