I don't know about magic bullets, am stiffer than I used to be but can still just about ease my way in and tell you that you need strength/flexibility in your hip flexors, hamstring and sort of lumbar area - yoga?
I am loving having some replies! I did a bit of gym as a child, lots of other sports. I started adult gym twice a week 2.5 years ago and have noticed that it is definitely a combination of strength and flexibility, I am also training for a half marathon so that is obviously impacting the flexibility. Just wondered if anyone had something that worked for them?