This is an excerpt from Running Well by Sam Murphy & Sarah Connors.
All in all, then, a simple and not-too-time-consuming warm-up is likely to make your running experience better. So how do you do it? Well, there are three stages: First, mobilizing the joints to prepare them for movement; second, gentle but progressive aerobic activity to start to raise heart rate and body temperature and increase circulation; and third, running-specific movements to prime the brain and neuromuscular pathways. To be effective, a warm-up should be intense enough to cause mild sweating, which indicates that the body's core temperature has increased by about 2 degrees F. Then you know the muscles will be ready for action. In a study published in the Journal of Science and Medicine in Sport, a warm-up that increased heart rate to 74-86 percent of maximum was found to be optimal, but a study on cyclists found that both high- and low-intensity warm-ups worked better than no warm-up at all in improving performance.
As a general rule, a warm-up should be at least five minutes long and could be as long as 20 minutes. If you are doing a short, fast session or race, the warmup should be longer and more thorough than if you are embarking on a slower, more prolonged run. This is partly because you don't want to spend valuable time getting up to speed in a shorter session, but also because faster running and racing puts you at greater risk of injury. You should also spend longer warming up if it's freezing cold outside, or if you run first thing in the morning, when body temperature is lower.
Don't allow too much time to pass between completing the warm-up and beginning the activity - otherwise the benefits will be lost. And as far as the length and intensity are concerned, make sure that your warm-up doesn't leave you in a fatigued state - otherwise it will hinder, rather than help, your running.
This is an excerpt from Running Well.