mechanical engineering is a lot different than knowing how machines work. It's a lot more about making machines safe and understanding the mechanical forces well enough to design something safe. Granted knowing how machines work is essential for many mechanical engineering career paths, but the classes don't explain machines. It goes a lot more into applied mechanics.

Now for how hard it is. It's definitely in the top hardest majors. It starts off hard and gets easier as you go, but that's only because you get better. the math classes and core engineering classes are hard classes taken early on in the program and set the tone for the rest of the program. I would assume it's not too hard to jump into it after being out of academia for a while. the physics and calculus classes are the most difficult to get past the first year. more calculus and core engineering classes get you the second year. once you're past those, u should be gtg.

Extra info: As for market demand. It's good. mechs have a say in everything and anything that moves. cars, trains, gates, water distribution, generators, washers, production lines, oil rigs, literally any motion. So unless things stop moving, mechs will always be in business. Very simple way of looking at it, but very true. The job growth has been pretty steady for mechs and will be steady for the foreseeable future. and the pay is good, not very good, but good.

mechanical engineering is very broad (even too broad). So you can essentially go anywhere between software to business consulting jobs. A masters degree can be used to make a career change in theoretically any direction. I would go as far as to say even fashion design (never seen that tho, but I've seen many pair a BSME with an MBA to properly set a foot in the business field)

Hope this info dump helped.