There is nothing wrong with running outside. Simply wear layers and take precautions when running where ice buildup is likely. I spent quite a few years running cross country at 8000' elevations for college, and outside for the vast majority of it. This included during rain, snowstorms, predawn in the dark, etc.
If you would rather run indoors, a treadmill is an acceptable substitute, or pool running is an alternative as well.
There aren't really any weight routines that will maintain/improve distance running fitness. The only way to really maintain running or improve it is to run. There are some studies that show plyometric exercises can help increase running speed, but these are mostly for the shorter (400m and shorter) distances, and are mostly related to the explosiveness of the start.
If you want to run at night, Petzl and a few other companies make very nice headlamps. I would get a headlamp that runs at least 40 lumens, a flashing taillight and a lightweight reflective vest. That's the setup I currently use when running at night.