Personally speaking I don’t trust the new plan builder for this type of event. There’s some good advice in this thread about AUDAX , whilst not exactly the same as a Gran Fondo (much less of a race focus) there’s some good points about volume and how people used TR for their training.
There’s an awful lot more to riding that sort of time and distance than just knocking out a couple of sweetspot and over/unders workouts each week, time out on the bike helps prepare you mentally for your long ride, you’ll learn what works (and doesn’t) nutrition wise, a bike that feels okay for an hour on the trainer is a different beast after a few hours climbing hills and indoors you’re not having to carry food, spares, clothing etc.
I’d aim to build up a long ride each weekend until you get up to about the 4 hour point, after that it’s arguable whether you are gaining much more that will help you progress and you are just accumulating extra fatigue, maybe aim for 2/3rds of the distance a few weeks before your event, just for mental preparation / confidence.
I wouldn’t worry too much about progression levels and all that jazz, being consistent in getting a couple of interval sessions in each week and then increasing your volume with zone 2/endurance riding will see you fine.