I like to wear things that I normally run in but are themed.... so something along the lines of capri pants, calf sleeves, and some sort of tank top. I really like the racer tank tops from Raw Threads and they have so many cute themed tanks! I have one that says "In a corral far far away!" because I'm a slower runner and embrace my slowness (I'm just getting better ROI for my investment!) plus it's a play on the "galaxy far far away".
I shy away from running costumes because I want to be comfortable when I run. 13 miles plus queue time is a long time for me and I want to be as comfy as possible without extra added weight. Plus sometimes the fabrics don't breathe just right. I'm about function over form.
January is a weird time here in So Cal - I live right down the street from Disneyland. We've had it in the mid-70s with full sun and some times it's cold and raining in January. This year has been abnormally warm but i cannot tell you what the weather will be like in January. Definitely agree with
@kirstie101 's suggestion of a throw away jacket or a mylar blanket. I've also bought knee-high socks from target for $1.50 and used those as sleeve warmers until I get too warm. They fit right up the arm and are easy to toss off when I need to cool down.