A couple of years ago I learned the hard way how to accidentally let your joints deteriorate--despite good intentions. For probably over a year I exclusively used an elliptical machine 2 or 3 times a week at the gym. I was in pretty good shape (cardio-wise) as a result but what happened was that my knees basically turned into moist spaghetti because I wasn't regularly putting some healthy stress on them through running (or something similar) on a regular basis.
So as a result when Agi and I went on a big hike all around Boulder CO for several days I injured my knee because it wasn't up for doing my "weekend warrior" activity. Since that time my knee has never been the same (I know, woe is me) BUT I am fighting back. I'm able to walk and hike no problem and I'm also running several times a week. However, the big breakthrough of the past month is: cross training. I don't even know if I'm using this term correctly but my recent discovery is that doing one kind of cardio-related activity is not enough for healthy joints. So I've started doing some treadmill running AND stationary biking. In the past several weeks my knee has gotten stronger and I'm feeling optimistic that good progress will continue.
Overall, what I think I've learned is that our bodies were not designed to perform well if we're sedentary computer marshmellows all week long and then suddenly want to go out and do something a lot more physical on the weekend. We perform best, I believe, in an environment that our ancestors lived in for a long time: constantly walking around on uneven terrain (e.g. in the garden of Eden or in the scenic hills surrounding our ancestors' caves). My legs and knees always feel great after hiking in the woods and if we could keep this up on a regular basis I think we'd be much healthier overall.
That leads me to a final thing I wanted to mention: in Portland, just a five minute drive from where we live is the biggest "forest park" within city limits in the US. It's a beautiful park--and an actual forest--and I'm looking forward to being a regular there.