I've done running in some pretty cold weather, and here's what i have found works best for me. This list is from last winter, where the temps here ranged from 25 degrees down to -40 degrees, windchill included.
1. Half Face Mask and fleece cap combination (can remove one or the other as needed)
2. Elastic Neck Gaiter
3. Full Face Mask
Remember to keep in mind that you simply CANNOT train on the same level in cold as you can in warm weather! It is dangerous to your health. If you don't have a heart rate monitor, get one. Focus on HR instead of your splits. And don't be afraid to mix up the cardio. Stationary reclined bicycle can be a good workout, ditto the stair master. During the winter I try to mix in a few alternate cardio methods, and my times sure as heck don't suffer as a whole. As always, YMMV