I have both bad back and runner's knee. Once I found out the most effective treatment for my bad back is regular exercise (40min a day, 4 days of the week... but I can't afford to skip it), I started thinking about other problems I could solve this way.

I thought about runner's knee. I started googling around. I found some contradictory info initially, but in the end there seems to be consensus squats not only don't harm a knee, they make it stronger! You're essentially building your exoskeleton.

What I found gives me best results is wall-facing squat.

It's difficult to start. It took me weeks, perhaps even months to feel benefits, but now I'm a squat addict. Wall facing squat is very good because it makes it impossible to cheat. You have to develop better stance, balance, and flexibility. And you have to think how to get there.

As a nerd who played computer games A LOT, I was so inflexible I could only straighten my leg about 120 degrees when lying on my back and raising the leg vertically. Now I can grab my heel with leg perfectly straight, or touch my forehead with my knee. So don't think you can't do that. You just need to be persistent, systematic and don't force it (especially if you're starting to feel pain).

Nowadays I try to incorporate those wide wall-facing squats into every raising movement I make. I get some odd looks (I don't do it at work, though), but I don't care. When I raise from bed, I don't use my hands. Same for couch, chair, and other platforms.

Don't get obsessed with numbers. Numbers don't mean a squat if they come at the price of bad technique. Bad technique is like building a tower on skewed foundation. Repeat that movement enough times, and the tower will start leaning, then fall (injury). So once in a while I lower my weekly number of squats, when I feel I can get even closer to wall. At first I thought I'd never manage it, but every couple of weeks I can exercising closer to wall. It's better to perform smaller sets of perfect exercise than lots of bad. Too bad I learned it the hard way.

Aside from feeling very flexible at thighs and legs in general, it's rewarding because my knee feels very stable. A few years ago, when I was shifting my weight while standing, I could feel it rotate in various ways. Now I feel more confident because I feel it supports me better. My balance also got a lot better and I'm leaning over tables and other objects in ways that seemed impossible before. I naturally adjust my stance when walking upstairs, downstairs, or running downhill. It feels like my entire leg works through the entire range of motion. I get bonus points when cycling, I can power through some smaller hills at full speed where previously I would be soaked with sweat.

In general, anaerobic exercise like squats gives your body more HP (Hit Points), making you more resistant to injury, increasing bone density etc. The last one is especially important for women, who are prone to osteoporosis when getting older.

I'm getting the itch to resume long distance running, and now I could probably pull it off. I learned to listen to my body, and prevent injury in advance by adjusting my stance and movement until I feel no pain or discomfort of any sort. I dig the endorphin, the runner's high. I don't know if i will find enough time, but worst case scenario is I will just focus on cycling.