I have never been on the cassin ridge, well beyond my skill set so take my opinion for what it is worth but
I would think that the best training for doing the cassin is to do lots of routes that are similar but smaller and progress closer to cassin in scale. that means doing a bunch of routes in the denali park.
how about start in the the ruth gorge area.
moose tooth ham and eggs
japan couliour on barill,
peak 11,300 sw ridge (probably the most similar but smaller in scale to cassin)
then some west ridge hunter
then up west butt or west ridge on denali to setup stash at the high camp for return and some acclimitize
run down to cassin and send? of course this would require lots of free time but imagine how cool that would be.