If cold weather below 40F is predicted throughout your race, you need to prepare so you can stay warm enough before, during and after, without overheating. You want to prevent hypothermia. On race day, you will want to stay with lightweight gear that will change your stride as little as possible. You want compact gear that you can carry easily, and preferably that is disposable so you can toss it if you warm up. To keep with the maxim that you use nothing new on race day, try these out during your training walks and runs to see what works best for you.
I find that my legs tolerate cold temperatures, but once it is below 40F I prefer to wear walking pants or tights. Tights are good because there is no extra fabric to flap around if you are going for speed. But since I am a walker, I simply love my Sporthill Nomad Pants in colder weather. But if rain is predicted as well as cold temperatures, you might actually be more comfortable in shorts rather than in wet pants.
I prefer a long-sleeved technical fabric shirt for cold weather walks. Once I warm up, it may be the only layer that I need. I also prefer one with a loose-enough sleeve that I can push it up to the elbow if I am really warming up, and long enough to cover my hands as a hand warmer. Whichever you prefer, be sure to select a sweat-wicking fabric, because believe it or not, we do still sweat even in the cold.
A windbreaker jacket is essential to staying warm before the start -- some races have an hour or more wait in the starting corrals or before the gun. If it is a chilly enough day, especially with wind, you will want to keep it on throughout the race. Some racers buy disposable windbreakers (seen at many marathon expos). If you want truly disposable, punch arm holes and a neck hole in a trash bag. You will see many other racers using this alternative. I prefer the Patagonia Houdini jacket, which folds up small as an energy bar but is windproof. Then if I warm up, I can just stuff it into a pocket or pack and have it ready for the finish or if I hit a colder, windy area.
I had an unpleasant surprise at a cold, windy race -- thigh chafing I hadn't experienced before. Protect your nipples, crotch, underarms and thighs with a lubricant such as petroleum jelly, SportShield, BodyGlide or other such products all areas that may chafe. I like to take along the SportShield towlette packets as they are easy to carry along and have handy if my preparation is failing.
You can keep your whole body warmer in cold weather by wearing a hat. I also find it essential to cover my ears in the cold. The simplest solution is a Buff -- the multipurpose microfiber tube you can arrange as a cap, neck warmer, earwarmer, or full-on balaclava. I never leave home without one in winter. The advantage of a Buff is that if it warms up, you can just wrap it around your wrist or stick it into your waistband. They are so lightweight that they are easy to carry along to have handy after you finish.
Your hands will likely be cold as you wait for the start, but may warm up fast once you get moving. I take along a pair of cheap gloves -- ones that I won't care if I lose one along the way. At any race you will tromp over the missing gloves of many a racer. A disposable alternative are worn-out socks, which you can wear as mittens and toss when done. For truly cold races, mittens are a better alternative for keeping your fingers warm, vs. gloves. As with your shoes, you can add extra insulation and windproofing by tucking a napkin into your gloves or mittens. I have also use the Grabber MyCoal disposable hand warmer packs on brutally cold days.
Winter races often start in the dark, and as you are packing for the starting line you may forget that you still need sunscreen, lip protection and sunglasses. Set these out the night before so you don't forget. The sun is even closer to the Earth in winter, and on a cloudless day you can catch a nasty burn.
10. Pack Warm Clothes
Check a warm jacket (or full set of dry clothes if it looks like rain) at the race gear check so you can immediately have them to keep from catching a bad chill. If they don't have a gear check, have a friend bring these to the finish line or have them ready in your vehicle. Be sure to bundle up in the heat sheet they usually give out at the finish. If they don't, have a friend ready at the finish with a space blanket and/or your dry gear. Your body will cool off very, very fast once you stop walking or running. You can get into real trouble if you don't get into dry and warm clothes fast.