You're ten times more likely to pull a muscle or injure yourself if your muscles are cold and tight. A proper warm-up is a vital part of your run, and it only takes a few minutes. You need to warm up all the muscle groups you'll be using, get your cardiovasular system going and up your body temperature gradually.
Warming up only takes 10-15 minutes and should involve a few minutes' rapid walking and then working and stretching your muscles whilst jogging (knee raises, heels to bum, skipping, and don't forget to a few arm rotations, core (trunk) and neck stretches too.