You don't need an excessive warm-up before running a marathon the way you might before a 5K or 10K race. Stick to your usual pre-run stretching routine, and if jogging around for 5 minutes or so helps calm your nerves, that's OK. But resist the urge to spend too much time warming up; you don't want to wear yourself out. If you are anxious, use the time to do more stretching instead. And especially in a large marathon, the start will be so crowded that you will be running slower than your ideal pace for the first mile or two, which serves as a warm-up. This can be blessing in disguise because it keeps you from running too fast at the start.