Had first lesson + video, any tips for flexibility?

Took my first surf lesson, surf was small. Surf coach let me try his 6'6 epoxy which is what you see me riding.
Instead of burning the awful and embarrassing film I decided to upload it for the internet to judge
Goals after this session/seeing myself on film
Posture:
1) Don't bend at waist, bend at knees and keep chest up
2) Bend back knee inward/towards nose of board and not outward --- contributing to poo stance
3) Use front arm to guide cutback towards foam (made progress in this department already immediately, see picture
4) Keep on the wave as long as possible (duration and flow)
5) get low for cutbacks
To be honest I think my issue is leg/hip flexibility. I don't understand how people can bend their knees inwards to such an extreme degree, especially the rear knee. Ive been watching videos and trying to increase flexibility at home. I feel quite a large pressure on difference parts of my knee when I do this, and to be honest it seems like a perfect way to tear a ligament in my knee, wtf?
Anyways, hoping someone can chime in with some advice!
Instead of burning the awful and embarrassing film I decided to upload it for the internet to judge

Goals after this session/seeing myself on film
Posture:
1) Don't bend at waist, bend at knees and keep chest up
2) Bend back knee inward/towards nose of board and not outward --- contributing to poo stance

3) Use front arm to guide cutback towards foam (made progress in this department already immediately, see picture
4) Keep on the wave as long as possible (duration and flow)
5) get low for cutbacks
To be honest I think my issue is leg/hip flexibility. I don't understand how people can bend their knees inwards to such an extreme degree, especially the rear knee. Ive been watching videos and trying to increase flexibility at home. I feel quite a large pressure on difference parts of my knee when I do this, and to be honest it seems like a perfect way to tear a ligament in my knee, wtf?
Anyways, hoping someone can chime in with some advice!