I find that being sedentary can get you stuck in a weird local maxima where your body hurts, but it hurts even more when exercising (back hurts, RSIs, etc) and so people don’t exercise and they protect their body like it is glass. If however, you can do some moderate exercise (swimming worked well for me) you can gradually build up your strength again and all of a sudden your back isn’t killing you anymore and you are able to strain you body much more without being in pain.

I personally went through this journey where I was sedentary for about 10 years and in my late 20s I could not run, I could not walk long distances, by back was killing me, I had wrist issues. Seeing a physical therapist who could help rebuild my body while starting with light exercise and gradually building up worked for me. I spent 2 months just strengthening my ankles and feet before I could run because they were so atrophied.

Eventually I got past that local maxima and now I can run 25 km with 2km elevation gain up a mountain and back which would have killed me before.

Lastly I will say that exercise should be gradually eased into, a lot of people are put off by exercise because they start too hard and make themself miserable. For cardio you should try to keep your heart rate within a lower range (zone 2 cardio at around 70% of max heartrate). For most people this means your cardio starts with walking up a hill, and you won’t get to actually running until later.