How far can you run on a treadmill?

Wow, it is more than a little cold outside.  Yesterday was windy and I was kind of glad that I was too busy with party preparations to run, but now I am regretting it.  I finally ordered a pair of running tights from SierraTradingPost.com.  They got down to $10 and I figured that even if they don't fit, I can't go wrong trying them at that price.

So, until I get my tights I am not sure I want to run outside in this kind of cold.  That leaves me with the treadmill option.  I need to get in a 12 mile run, so I wonder if I will go insane on a treadmill for two hours.  They do have a TV on each treadmill at my gym, so maybe I can find a good Christmas movie to watch if I decide to go that route.  I am going to see what the temperature is at 11:00 and make my decision, but I am pretty sure I will be on the treadmill...