From what I read in AWP, the senshusei boyz are cannon fodder for the BIG DOGS at YOSHINKAN HOMBU. Of course, Mike Twiggy's description of the course has been challenged. So FWIW...
To answer your question specifically:
Q: just how physically demanding it is and what level of fitness is required?
A: Very physically demanding. Think of your previous Gasshuku (Intensive) training, now multiply it with 365 days 24/7. Get the picture.
Q: the daily routine
A: It is a form of shugyo (monastic endeavour) It is a detachment from your everyday life for 365/ (366 if it is leap year) days.