Living in a country with a long winter every year makes it impossible to stay inside when it is cold and at the same time maintain the good shape a marathon runner needs to be in to be able to call himself a marathon runner. I live in Iceland. Of course I could try to use the cold weather as an excuse to stay inside, in my warm cozy home, and not get out there to run. I know some people do. And I guess it is quite understandable if you live in California or Italy if you do not see any reason to run when the temperature is below the freezing point. Or if you come from a climate where the temperatures rarely go under 20°C, then I guess it must be quite uncomfortable to be out there in -2°C.
There are other things that can bother a runner during the winter in Iceland. It gets terribly dark. For at least two months it is already dark when you go home from work, and of course it is dark when you to to work. If you don't have the opportunity to run in the lunch hour, then you can forget all about running in daylight. And then there is the snow. Sometimes all the tracks and paths are covered in thick layer of snow and sometimes there are even ski tracks on top of my favorite running paths. Then you have to find another route.
The most dangerous thing though is the ice. The slippery spots you can't see because they are covered with snow. If you step out on one of those it is very likely that you are going to fall. Hard. I was once in great hurry downhill. There was a bend on the road and I wasn't aware of the slippery spot that hid down there, under the snow. I think my feet flew higher than my head. And I got a very clear and big blue spot on my hip. Luckily I didn't break anything. But it hurt! And I was startled. I could continue on my route, but I assure you I am very careful when I run past that spot ever since. Even if it is in the middle of the summer in bright sunshine. It is easier if the ice is on the surface and you can see it. Then it is easier to avoid the icy spots. And there are different kind of shoes one can get that are quite suitable for running during winter time. Just as there are different kinds of tires for cars. You don't have to have nails in the tires to be able to drive on snow or ice. And it is possible to get shoes that are in fact pretty good on ice. Not slippery at all. If the ice isn't too uneven or wet it is possible to run just as one would to on a firm surface like asphalt.
Sometimes there is a thick layer of snow everywhere when I get out in the morning. Then I have to use the tracks from the few cars that have been around, because at that time the big machines that clear the snow from the streets can't be everywhere at the same time. And of course I have to be careful not to let those machines run me down. I suppose a lone runner out there early in them morning can be difficult to get an eye on. Which leads me to another thing. Reflective bands and vests. They are as important as the shoes in the dark winter. If the light is changing and maybe a little rain it can be almost impossible for the drivers of the cars to see a runner in black clothes. And somehow running clothes seem to be black. It is possible to get all kinds of bands, vests and those reflective danglers you just can put in your pocket during daylight and then let hang down on your side when it is dark. Everything helps. I find the vest easiest and best.
It can in fact be quite pleasant to run in the snow. Hard packed snow that isn't too slippery is great surface to run on. And when there is snow on the ground everything looks very clear and clean. I like the crisp cold air in the winter even if some people do not like the thought of breathing very cold air into the lungs. I have, as a matter of fact, never had any problems with that, not even in -16°C as I sometimes have run in. I just take good care to cover as much of my skin as I possibly can. Zip the jacket all the way up, wear an elephant hat and gloves. Sometimes an extra scarf. A thin woolen sweater under the jacket. And two pairs of pants. That does it. When it is very cold the weather is usually still and not snowy. The worst conditions I can think of are when there has been frost rain. When the earth is frozen and it rains. Which means the raindrops turn into ice when they reach the earth. Then everything is covered in thin layer of ice. Which is almost impossible to run in. I also find it difficult to tackle very wet snow. When the snow on the ground is melting rapidly which means it is very wet and splashes everywhere. It is possible to run in it, but no fun at all. In fact it can be extremely irritating because you get your feet wet in seconds.