Just thinking aloud... if you offer people several work options, saying they have to do one of them for a month, and get the same salary regardless of their choice... and most people avoid an option X... and it's not specifically because X is seen as low-status or physically dangerous... then X likely is "hard work".
Also, maybe it is necessary to distinguish between "works that are hard when done properly" (but maybe a lazy person could make their job easy by cutting corners) and "a person working hard" (but maybe because they chose to do more than would be normally expected of them). To use one of your examples, going to gym is hard work compared with sitting at home watching movies, but someone who only goes to gym to ogle girls is not doing hard work.
I suspect "hard work" will be relative to who is doing it, but if you don't mention who is doing it, then an average adult able-bodied person is assumed.
The concept of "hard work" seems very confusing to me.
I think it could mean combinations of many different things. It doesn't seem that helpful for actually carving reality at the joints for engineering purposes and getting better at whatever traits are needed. It seems like a messy construct composed of many dimensions, and in fact the dials on those dimensions could be set to very different settings and still be called "hard work" without distinguishing between the different underlying factors.
I want to be able to reductionistically decompose what another person could mean when they use the phrase, so I could then narrow it down to the precise anticipation of experience that they are implying. I have some guesses as to some of those parts below.
What I want is a list of Hard-Workingness Theories, composed of particular precise-as-possible lower-level pieces, rank-ordered by plausibility/usefulness/interestingness at capturing what you think people in general mean, and/or your best attempt at defining it. Maybe there is some general feature that neatly explains most of it.
Spoiler alert if you want to think before seeing my thoughts (also, apologies for redundancies and bad grouping in my list). If someone wants (someone else) to work hard, it could mean any combination of the following components:
There are various examples that could be classified as "hard work" or "not hard work" depending on what exactly is meant by the example and the particular profile of meanings a hard-workingness classifier is using. One can test their theories of hard-workingness on these examples and ask, "Once fully specified, when will or won't this category be hard work, according to this hard-workingness theory?"