With my gf, we've not found a 40 mile route there from NR2, we often do a 48 but have done a 60 there. The heat made it harder. We did stop for a picnic on the beach and then did similar on a ride to Winterton on Sunday.
We do long but not fast rides, averaging 13 - 14 Our first century, the Norwich 100 this year, didn't scare us as we were only aiming to finish and had done multiple 90ish mile rides - although I nearly didn't make it back from Bury St Edmunds one day.
Having said that, we have thought about moving to road bikes and seeing you disappear didn't diminish that thought!
I can see that about round the castle, although it tends to be mid week mid day when I do such a route and then with shoppers and office workers on lunch stepping out it's not really the same!