I’m planning on walking the CP late December through January. Any pitfalls for that time of year? Things to coinsider? Best route from Lisbon?
Just a warning--that is the time of year, for that area, that has the absolutely worst weather. We are about 100 km north of Lisboa, and subject to the same weather patterns.
This November through late January was the first time we'd stayed in Barquinha over Christmas and January. It was cold and rainy most of the time, with the occasional sunny day. Also, for the first several days (walking-wise) out of Lisboa, there is little or no shelter, and few stops available, during the day. You might want to consider a different route, if that's the only time you can choose.
Otherwise--serious rain gear, umbrella, waterproof boots/shoes as you'll be walking through agricultural areas. And it can be quite cold as well, so maybe merino/similar undergarments. The older Portuguese buildings have very little insulation, so your nights will likely be coolish, as well.
Sorry to be such a "downer," but of our times spent in the area, maybe 10 longish trips over the last 5 years, this last one was the worst, weather-wise (though great in many other aspects .)