I’ve been running more lately and have been really focusing on asanas that assist in stretching out the hamstrings and side body.  My go to pose to target these areas is Trikonasana or Triangle Pose.  I absolutely love to do this pose after a run.  Depending on the depth of your angle in this pose, you may also feel a stretch in your calf and groin, which is also really beneficial for active runners as those areas tend to get tight when running a lot.  Here is a great article about Trikonasana on Yoga Journal.