whether or not it's good to go into a deep stretch before your body is warmed up is debatable. as a beginning runner, i used to dive right in to a hamstring stretch before i'd start running. i'd throw my leg up onto the railing of my front porch and lean in, feeling that awesome stretchy feeling. i'd even wake right up and sit in half lotus and journal before my run ... which i think ultimately could have contributed to some serious hip issues i experienced last winter (half lotus stretches the hips in major ways ... and it is very asymmetrical so unless you are switching legs to balance out, parts of each of the hips are gonna be worked a lot differently and stronger than the other parts).
i've found that doing a simple routine before i run has been quite effective (see my "morning routine" post ... ). i like to do a prone (on the belly) posture such as cobra (with gradually widening legs - 4 or so reps at 4 -5 different widths) to stabilize my sacrum and warm up all the muscles that weave through that crazy bone - i think this is especially good to do before running trails or other unpredictable paths that call for stability.
this is a great warm up posture, too! also featured in runner's world:
Quadruped: Position yourself on all fours. Raise your left arm and right leg until they are in line with your trunk and hold for 2 seconds. Return to the starting position, then raise your right arm and left leg. Repeat 8 to 10 times on each side.
[in viniyoga this would be done inhaling and raising/extending the arm and leg and then exhaling either into child's pose or table, then inhaling and extending the opposite arm and leg, all the while keeping a strong core and making sure that the hip of the extended leg doesn't raise higher than the other - level pelvis! ... called "sunbird" or "two legged cat," although i've never seen a cat do this! meow!]
I'm running my 7th half marathon on sunday, 2/14 in Austin with my brother. This one will be a pleasure run - gonna take it easy and be really really thankful that my hips are "happy" and allowing me to run 13.1 miles!