I had a neighbor who walked quite a lot. She wasn't young but able to do so. I joined in for a number of years and would walk about 3 miles a day. On other days I would bike or swim. Exercise is good but too much of anything is not. Unfortunately many of us don't know our limits. Maybe ego is involved. Yet we try and do the right thing. I had some warnings as my knees started to hurt. I avoided distance walking and began 20 minutes of walking sprints. Seemed to work for a time.
Then my hip started bothering me, to the point where it hurt to tie my shoelace. The owner of the vitamin store I go to had hip replacement from too much football as a youth. He told me what I had was the beginning. Too much biking, wear and tear. Later he eased his stand on that. Funny how so many of us who tried to do the right thing found the right thing turned and bit us. Anyhow, the cause could be overuse as mentioned, or could be from an inflammation, the latest phrase describing bacteria gathering at a weak link in the body. They get into your bones so to speak. Maybe it even related to bacteria from infections in root canals although hard to prove. In any event I've adjusted to being conscious of doing exercise in a smaller way.
My first observation is that we spend a lot of time on the computers. As such, it's good to get up often, say every fifteen minutes and do something else. Paperwork, picking up a piece of paper, washing dishes, organizing stacks, household maintenance, all require varied movements of the body. They don't ground and pound with one repeated motion the way walking or biking does. Each household chore is unique, slightly varies and uses slightly different muscles. So my new way of looking at getting things done around the house, even cerebral things, can be a form of exercise. Projects mean going back and forth, picking something up at point A and bringing them to point B. All of the motions use muscles and ligaments and reflexes. In other words, keeping your life going is the workout, and the home can be the gym. One is not always conscious in this way, but it does creep in.
I still go out and walk or bike, but its not a 'have to.' In addition, exercise can tire you out and your energies aren't going into getting your stuff done. So this is just another angle, another perspective on using your body in a smaller way but a good way. Nothing is the answer in and of itself, but little tips and observations do help out.