Haven’t ridden an exercise bike so don’t know how relevant they are. I’m presuming you’re thinking about snow riding. (Fatbikes are great almost anywhere though)
However I have ridden extensively in deep snow where the pedals are scooping chunks out each rotation, and it’s like riding in molasses, so I suspect your regime is probably ok.
Just need to step it up to a few hours if you want the fitness to actually get anywhere in deep snow. 3-4 miles can wreck me if it’s deep enough – but it’s great to be out in it.
Snow that’s not deep enough to be scooping is not much different to any other soft surface IMO.
Crusted snow can be faster than tarmac, but there’s always the chance of breaking through. 🙂
That said there’s about a million different types of snow, and there’s no perfect method or system.