Working as a dev doesn't inherently prevent one from being healthy and active enough to be able to run a marathon, just gotta spend some time being active in your spare time. But killed knees and back due to a job in trades certainly will prevent you from running a marathon, no matter what you do with your free time.

Sitting in a chair all day is horrible for people though. (To say nothing of what it does for your attitude and personality).

You can easily mitigate that by making adjustments to your workplace (adjustable sitting/standing desk), as well as by avoiding spending a lot of time in a chair and being active outside of work. With trades, you cannot really do anything outside of work to successfully mitigate the damage done to your body, unfortunately.