I like what the others have said. I looked up a couple of plans:
Higdon’s longest runs are 20 miles (32km) - as his plans get more advanced he does more of them, but doesn’t go over that distance. He says run your long runs 30-90s/mile slower than your target race pace.
Magill (Build Your Running Body) uses time not distance. His longest run is 135-210 minutes. For me, his pace would be 5:46-6:39/km (this is based on my 5km time, which in my case is 21m flat). So if I ran the full distance at the top speed, I’d hit around 37km I guess.
I think your plan sounds sensible. My half PB is 1:37 which is 4:36/km or so, and my coach predicts my marathon potential at 4:53/km. So you might find you could go even faster if you trained well (but I’d suggest starting steady, and going harder in the last 10 if you’ve got it in you).
There are race pace predictors which might help you, as well.