Yoga... and more yoga.
If you're a runner, you get the joke - our flexibility is not up to par to say the least. However! The benefits of yoga are literally endless: anxiety reducing, improves overall fitness, increases self awareness, boosts metabolism, improves flexibility and breathing ...you get my point.
My favorite and go-to yoga videos that focus on running benefits and improvements are listed below. If you give em' a try let me know what you think!
This article from Harvard Health also goes into detail about yoga benefits and more information.