Years ago I developed a condition that prevented me from finishing any race longer than a 1/2 marathon. In fact I DNFed a few of them. I would get sharp pain in the back of both knees every run. I went to a PT who dx'ed tight hamstrings and recommended a daily stretching routine (the door frame stretch). Since that day I've done innumerable 1/2 IMs, IMs, marathons and ultramarathons, all with no knee pain issues. Does stretching work? It does for me.