When it comes to endurance training, there are no adaptations induced by duration that aren’t induced by intensity. It’s only a matter of the total “dose”.
As to what “dose” is optimal, I don’t think science can really answer that question. As a half-century student of endurance sport, though, my impression is that things start to saturate somewhere around 15 h/wk.
IOW, if you’re only training 10 h/wk, then most individuals could probably get better by just training more. However, if you’re already doing 15 h/wk (on average, so 700-800 h/y), then I’m not so sure.
I would agree that you design a training program by penciling in the key (most intense) workouts first, then flesh things out with “moderate intensity filler workouts” (as they are known in our household) to the limit of the time or training tolerance available.