When it is really cold, I just use more than one long johns. I have a 3/4 one that come just under my knees to keep my hips extra warm. And a long one on top of that for the whole legs. And sometimes a third long one, if it is really cold.
I need to warm an extra pair of trousers when the temp is below -8C. When I used to walk to the train station, I use to put rain trousers over my normal trousers. Those were easy to take off when going on the warm train. But as long as you stay outside, something under your trousers might be more convenient.
Protect yourself against the wind chill. Wind tight trousers and jacket, something around your neck, a woolen hat, gloves. A jacket with zippers under the arm pits give you the option to open up when it gets warmer. Bring along a warm down jacket in case you want to sit down somewhere, and a foam plate to sit on. Warm tea in a thermo bottle will also be nice.