You’re right about the sustained power build workouts. I go for them too.
A friend told a story about what happened to him and a fellow randonneur. They were cruising in to the finish of a 300 km, still had 20 or so to go. A young kid buzzed right by them. Didn’t warn them, didn’t greet them, just whizzed by.
They looked at each other and pulled him in. When they got up with him, they started a pleasant talk about the wonderful cycling events called brevets. All the time picking up the speed a little at a time. He was hurting. After a while, while they still picked up the speed and babbled about how cool it was to ride long endurance rides, he suddenly declared that he had to turn and pulled off onto a residential side street just to get away from them 
Yep, TrainerRoad helps. I’ll be following the same strategy as you. Sweet spot and then Sustained build. Fantastic value for the money.