We've had a cold and snowy winter. And I do not treadmill it. Just no.
I have no trouble getting all my miles in. Some are slower than others, thanks to fresh snow or slippery footing, but it's pretty easy to get out there and do it.
With my workouts, I have to be a little more adaptable. I skipped a workout this Tuesday because we had just gotten 5 inches of fresh stuff the night before and there was nowhere with good footing. I moved the workout to later this week and it was great.
As others said, look for places that have some consistent clearing. There are a couple bike paths near me that get plowed (or sections of them get plowed). I do my workouts on these, and sometimes it makes for quite a few out-n-backs. I also tend to make these workouts (the shorter ones) time based so I'm not overly stressing about splits or anything. So instead of 800s I might do 3 minute intervals, or whatever.
So yeah, my advice is get in the miles, do the workouts when you can and don't be afraid to shift things around. Skipping a fast workout and just doing a steady 12 miler in the snow still makes for a really good workout. Especially for a marathon still a few months away.