There is a lot of controversy out there regarding stretching. Should you stretch before should you stretch after? After much research and training, I believe stretching is important after your workout not before.
It's important that your muscles are warm when you stretch otherwise you could end up pulling them. If you prefer to stretch before you run, warm-up before the stretch with some slower walking or other low impact movements such as marching in place or side steps.
Stretching can help lengthen your muscles and brings blood flow to the muscle to help it recover and repair. It can also improve your workouts and keep you from being too sore.
Most of all never, ever stretch a cold muscle.