Sorry so slow to respond. I saw this and then forgot about it.
So Coach Andrea recommends the serious Italian to all his athletes averaging at 6 hours a week or more for a build period.
So it depends on how many week you have left until your event but generally the specialy phase or what we call the peak phase is mostly about your long ride. Especially important for the Gran Fondo. In general your entire season should be focused on the long ride but it is much more important in the build and specialty phases. Everything else is icing on the cake.
Although TR’s plan might work fine for some people I don’t think that is the best suggestion. One long ride, one threshold and one strength endurce each week gives you that best diversity. The three are very different. We have on peak block with dynamic force in it. That would be my favorite and Andrea’s recommendaion. It would give you both your strength endurance as well as some of that higher intensity.
Other than your 3 core days the rest should by Z2 or Z1.
We have multiple people reporting the best results they have ever seen using the serious italian block for a build phase. One said after 3-years of TR they gained more/faster with 11-weeks of the serious italian then they ever did in TR.