Long Run on Treadmill - Thoughts

In school right now in upstate NY, there's been a lot of snow recently, and temps have been down to the single digits. I can work with that and don't really mind it, but the ice is another issue. Last week I was stubborn and did my long run outside, only 8 miles, still very early in my 50 mile ultra build, about 18 weeks out, that run sucked, I was a minute per mile slower than usually and felt like I was running in sand that was slippery? This week I've been on the treadmill 3/5 days. This morning I'm outside and realize this town doesn't shovel their sidewalks, all the sidewalks are coated in snow and ice to the point where it's on the same level as the snow on the grass in front of the houses. I have a 10 miler tomorrow and am heavily. considering doing it on the treadmill, thoughts? What would you do? Am I being soft, or do I need to put my ego on the shelf and thug it out?