I think you're underestimating what it means to perform a physically demanding job. It's not like that weekend you spent painting your appartment. It is something you do five days a week, for weeks and months on end. In my late thirties I was sick of being a software developer and I did a two year stint of being an electrician for a big construction company. It was a wonderful experience and I learned a ton of stuff about a ton of stuff, but in the end I was very happy to go back to a heated room where I can work sitting in an ergonomic chair while sipping a cup of coffee. Honestly, if your body was not trained to do this from your late teens or early twenties you will likely not last.