Good article. I've heard in the past that cartilage can heal but for some injuries it's just not practical. Some meniscus tears can heal but the recovery takes a year of slowly increasing activities.
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.