The Marathon Winters are harder work even when pumped up to 70psi for use in the dry.
I let a lot of air out of mine last night before I took them out, I'd reckon they're probably around 15 to 20psi now, which is probably the sort of pressure you want for cycling in significant snow, although it doesn't make them any less effort.
Both Zev and Kai have come back in, they've had enough of the snow now! Kai was in faster, although given his more mature years, I'm guessing he's seen it all before. Since Zev is smaller (ie she's got a reduced volume to surface area ratio compared to Kai), and with a much lighter fur coat you would have thought she would have gotten colder faster, but maybe the novelty won out!
Edit: Famous last words, a beep from downstairs tells me that she's just gone out the cat flap again, Kai is sat by me, so I know it's not him!