Good to hear you found the mwod.
Would be a good idea to start from episode 1 and move through each episode especially where you are “terribly stiff and inflexible.” As you start working through things you’ll see other aspects improve from working within the system.
One to start with
Episode 13: It’s Just Movement, Break it Down
A couple on sitting. There are more episodes on sitting.
Episode 88: Desk Athlete Hip Rescue
Why Sitting Wrecks Your Mad Hip Action
Episode 15: You Must Defeat The Evil Chair Part 1
Episode 15: You Must Defeat The Evil Chair Part 2
Episode 187: Death by Chair. How Much Do Actually Sit?
Episode 91: Mobilize In the Position of Death by Chair
Episode 55: In and Out Your Hips, For Points!