I have a gym in my basement with a treadmill.  I use it when the weather is nasty, but I think Maryland weather is never as bad as Wisconsin weather.  That said, its always better to train for a road race on the roads.  I like to say I don't do lightening and I don't do ice.  Use the treadmill sparingly, remember your marathon will be outdoors.

I'll run in temps down into single digits.  But icy roads are an invitation to injury.

The marathon is a very mental event.  You best train your mind for it by training under circumstances that are closest to what you expect for the race.  The variable weather you encounter in training toughens you for the variable weather you can encounter at the race.