I'm a bit hesitant to answer questions, as I'm only a "transmitter" of information. But to be brief:
It's more like starting with moderate workouts and progressing with them. 8x400 at 1500m pace with 2min recovery is not so hard, but once you've done that, you can do (for example) 8x500m or 8x600m at the same pace. Is this harder? Sort of.
Extension of race-specific speeds and modulation is the name of the game in the "specific period." Canova DOES believe in big stresses, but big recoveries following them. When we start talking about "runner x doing y miles a week, etc etc" I think we risk missing the forest for the trees.
As far as mileage, it's not so important in the specific phase. The only reason you go running at ALL between workouts is for recovery. The 2-3 days of regeneration running between workouts is NOT training, it's recovery. So whether you do 50 and 20 or 60 and 30 or 70 and 40 minutes of running on those days is probably not too relevant. But maybe Renato Canova himself will show up and tell me I've got it all wrong!