Try asking truck drivers - they're really helpful and there are many of them traveling across Poland.

The hitchhiking isn't that difficult considering there are still lots of (mainly) students hitching here. People are still quite used to young people hitchhiking compared to some places in Western Europe. Having said that, it will most likely be cold (especially in the northern half of the country) and slow (again, especially in the northern half). A lot of people that will pick you up outside the major cities will likely not speak English. As long as you stay along the A4 corridor, you should be fine, even if you're not that experienced of a hitchhiker, if you want to hitchhike around Podlaskie, just come prepared, or wait two more months...

It's not such a good idea I think, unless You are a survival type and you don't mind freeze, snow and darkness :)

This is not the best idea

I think it will be very cold and dark

Poland is not the best country to hitch-hike in January. The temperature is usually below 0 Celsius degrees and it is snowing or raining or both ;)

and the days will be short; hitching is difficult in the dark.

Poland is very cold and snowy during winter holidays and you may get stranded in the bad weather.