You would think running in 5° would be cold.  Certainly, less than comfortable.  I ran in 5° this afternoon and can tell you it was really quite nice.  Full Colorado sun.  Double sun really if you count the reflection off the four inches of fresh powder.  Absolutely zero wind.  The cold is just better in Colorado.  I ran five miles today, layered for sure but no balaclava or anything extreme.  Felt great.  Might get into the double digits tomorrow.