In reply to Becky:
Varying thickness tights help a lot, but I tend to underdress legs as it’s trickier to change them.
Tops wise, I find a long sleeve shirt with a zip to be most versatile. OMM Vector, Montane Dart, etc. You can zip up and down, as well as roll up the sleeves on ascents. If it’s chilly, a tighter fitting T-Shirt or long sleeve one does work wonders.
Then if it’s windy or cold, a wind proof is great. As long as you’re moving, you are generating quite a bit of heat and wind proof helps to keep that in.
Waterproof jacket and trousers are great. Montane Minimus are great all rounders running wise, Montane Podium is good if you are expecting lighter rain when it’s not so cold.
Hat, headband, buff and gloves are a must depending on weather. Luckily they can all be found quite cheap and having a variety is good.
Don’t discount socks. Wool socks are great as they keep feet warmer. You’ll get wet feet and keeping them warm helps to feel warmer overall (also it sucks less when you eventually get a low point and cold).
Then carry a synthetic jacket for emergency and a survival blanket or bag. I would recommend a bag as some races require one and if you ever need to use one might as well have something more substantial.
You don’t need to overpack, but you eventually learn what you need and what becomes handy. I did a hill run with club in November, got my gloves wet (no spares) and not a substantial jacket. Combine that with a lot of stop start and you get quite uncomfortable...
P.S. Some hot tea left in the car is not a bad idea either.