You can toughen up your knees by gradually ramping up the stimulus over time, and high impact sports of youth won't necessarily prevent one from being able to run a marathon in their 70s.
I also started from the Youtube videos but I got a lot more out of it after I started doing the full program with coaching.
i am "scared" because this guy stands most of the time, walks to work and back, does most of his work standing. i on the other hand either sit on my chair slouching for 12 hours at a time and if not there, i sit lounging on the floor and do a reclined L with the most pressure on the small of my back.
i am concerned because i have had back trouble for the last decade, usually it was kidney stones but later because of my posture. i hope my back is right but i wouldn't want to subject myself to surgeries. shit.
Its incredible how much sitting you can compensate with that sport.
Start slowly with at home exercises and work your way towards a healthy back. https://www.youtube.com/watch?v=QNgqDt_nMVQ