With the proper winter running gear 11 degree wind chill is not too cold for outside..just sayin'
I too am much faster outside and my stamina is much improved. A good day on the treadmill is at least 15 seconds a mile slower than a bad day on the road and in some cases a full minute/mile slower. As far as distance, I can do 10 miles outside with more ease that 5 inside. I'm sure part of the reason is mental, when you are staring at the numbers it is too easy just to think about the fact you are running. Outside you are concentrating on that next tree or stop sign and can loose yourself in the running.
I despise the treadmill and do everything I can to avoid it. The vast majority of the time it isn't the cold that forces me in but icy conditions that just aren't worth the risk. A couple of things that help me get through the indoor runs are:
The gym I run at has a theater room that is a bit darker and has a movie on, it is a good distraction and the darkness makes it feel cooler.
If there is a fan in the room and no one is using it I make sure it is blowing on me to cool me and simulate being outdoors.
I never allow the incline to go below 2. This makes up a bit for the fact that on a treadmill you aren't using any muscles to actually propel yourself forward.
I will ratchet the incline way up from time to time to simulate a hill. It also makes the 2 or 3 I am usually at feel flatter after that "hill".
Make sure you don't neglect outdoor running completely because the muscles you use on a treadmill, and the way you use them, are very different than the way you use them outside. Once you get a little used to outdoor running it isn't as bad as the first miles seems. I've learned the hard way over the years that it is better to be a bit cold the first mile and warm up than be nice and warm right out of the house and then feel like you are suffocating. Good luck with the training.