Parvottanasana (Intense side flank stretch) is indeed intense and thorough in opening your body and mind!
To build the actions needed in the legs, play with a chair!
Start by rooting your heels in Ardha Uttanasana (half intense forward extension)
Then use a chair with the backrest on the hip crease to "levelize" your hips and seat the thighs back as we practiced yesterday in Ardha Uttanasana at the wall.
Using the chair you can walk your hands down towards the floor extending the sides of the chest fully.
Now try the full pose with Paschima Namaskar arms. Use the hands at the back to work on extending the front body.