Many blue collar professions tend wear out one or more essential body parts in some manner regardless of cardio or strength fitness. Maybe some are differently "easier" than others like HVAC or electrical, but they still take a toll on knees, necks, and backs that can render one incapable to perform the tasks. Some guys last longer than others but there's usually a decision point of retirement balancing enjoyment vs. additional income vs. retirement health.

My dad was a light duty mechanic with his own specialty shop until 1986. He blew out a cervical disc and exposed himself to a variety of carcinogenic chemicals, and that was the end of his career.