I'm a paleo / primal nerd and they don't discourage cardio. Our ancestors were runners… so feel free to run. It's one of the reasons we don't have any hair. So that we could do cardio and have evaporative cooling keep us from overheating.
It's persistence hunting. There are tribes in Africa that still do it… basically chase an animal until they overheat :)
HIIT is different in that yeah, long duration cardio is important but people did HIIT activities tool. :)
I was joking when I said insane. It's just that most people don't backpack in the winter.
If you have the RIGHT gear you should be plenty warm. It's just that most people don't do it with the right gear so they feel miserable.
I think the hot tent that I am setting up will be a big game changer. This way I can spend those hours in the tent without worrying about being cold and can just relax in my hammock :)