This may sound strange in a collection of tips touting the benefits of high-tech sports apparel, but experienced runners know that a big, plastic trash bag is the best (and cheapest) disposable windbreaker you could have. Most marathons and other races start in the morning, and sometimes it's chilly enough that an extra layer would feel good for the first few miles, but then you will be too hot. No one wants to throw away a favorite moisture-wicking jacket or vest, but a trash bag can be discarded at a convenient water stop along the race route.
Here's how to do it: Just punch a hole in the bottom of a large trash bag (I don't receive any funding from Glad or Hefty, but I have used both of their products). Put your head through the hole and punch out armholes at about shoulder height, so your arms have the full range of motion. Once you feel warm, whether it's after 1 mile or 10 miles, just grip the bag at the neck and rip it down so you can shrug it off without having to pull it back over your head while you're running.