Running in an old worn out pair of running still gives you more cushioning than desired, as I discovered Jan 20th after I slipped on a pair of mukluks and went for a 6 mile run. This felt like running in barefeet for the first time all over again. I haven't worn the shoes since (except for one race on a river with choppy chunks of ice). The mukluks are too big which I didn't notice until running in them. Plus the pom poms drive me a little batty.
This routine works for me:
0C to -20ish with cleared pathways:
Thin layer of socks, Gortex socks, followed by wool thicker socks. Swim shoes.
-25ish ( depends on the windchill) and Colder OR running in lots of snow without cleared paths:
swimshoes inside a big pair of socks, mukluks ( so they fit better). Take a big piece of leather lace and wrap it up the calf of the mukluk to prevent pom poms from moving around, and reinforces the mukluk from slipping down the calf.
Wet Weather +2ish:
When your run consists of constantly running through puddles from all the melting snow, running shoes are best. They elevate your foot giving you a better chance of staying dry. Don't forget the Gortex socks. This kind of weather doesn't usually last long so in a few days you can be back to running free again.