Mark - Keep working on the 3 ball exercises I have you doing....that should eventually take care of the majority of the dynamic flexibility work you need.
My general philosophy to stretching or flexibility is that there is usually a reason why runners have tight muscles. And if these tight muscles are above and beyond allowing the runner functional range of motion, I then look to the cause of the tightness. Typically tightness is caused from over use. This over use can be the normal over use from repetitive running or may stem from muscle imbalances. I great example of this is tight hip flexors, gluts, TFL, and IT bands. This is very common in runners and static stretching doesn't do much good here because you need to address the muscle imbalance or improper firing sequence first before ANY type of flexibility work will take hold.
This is why you may have never experienced much improvement when stretching. So again, with the exercises I have you doing, we are working on the of your stability and strength of your hips/gluts/hamstrings while at the same time getting a dynamic stretch.
One thing to keep in mind. Flexibility is vert important, but a certain amount of muscle tightness is required to be fast!