Back on topic... you've already mentioned the weight, the pain, the VA pills. Do you know what's actually wrong with the knees? What kind of pills did they give you?
As to MMJ, it's my semi-uneducated opinion (no direct knowledge) that it probably does help with some chronic pain. It wouldn't necessarily be my first choice but certainly worth a try when other meds haven't worked. At one point when I got hammered by rheumatoid arthritis, I thought I might have to consider some form of MMJ, but other meds got it under control. As for being OK because it's natural, well, poison oak & rattlesnakes are natural too.
Depending on where you live, it can be a legal gray area -- even if it's state-or-local legal, the feds see it differently. Doesn't help that so many recreational users & profiteers hide behind a 215 card, it only makes it worse for people who can really benefit from it medically.