Dont stretch before hand. stretching a cold muscle will NOT make some kind of fluid go into it, but it might damage it.
Muscle receptors measure both the amoun of stretch, and the speed of stretch. By holding a static stretch, the speed is 0, therfore you are conditioning your muscle for a static activity, then immediately doing a dynamic activity (running). This doesnt make sense, and is not a warm up at all (when holding these stretches do you produce warm sweat?)
The best warm up is joint rotations in every joint of the body, in both directions - this losens the joints and produces synovial fluid which lubrictaes them. If you must stretch, do dynamic stretches where you swing/raise each limb in a controlled manner to feel a dynamic stretch. Dynamic is different to ballistic (throwing your limbs a high as possble outisde of msucle control - this could cause injury)
After your run, this is the time to do any kind of static stretches, when the msuclkes are warm, and you are using it as a cool down rather than a warm up. The stretch will improve recovery and facilitate waste product removal from the muscle, as wll as increase the muscle length. Hold each stretch for 20-60 seconds.
Thanks
Alex