I knew of someone (not personally) a friend went to exercise class with who was this type of person you mention. I did not know her, but knew who she was.
What I mean by that is, aerobics class, on the stairstepper for an hour or so, treadmill, etc. No weights though. No free weights. No nautilus machines. Only aerobics or aerobic type exercises. This woman exercised basically non-stop. This was years ago. At that time she looked like a walking skeleton. She was really thin and her skin had that yellowish gray cast to it, and she stunk.
About a year ago I saw her out with her walker going for a walk down the street. I assume her body had been pushed so far over the years that it had finally just given out. It is really sad to see someone do that to their life.
Here is a clue though that I have picked up on over the years, because this is not the only person I have known like this; when a person goes to a health club and exercises like mad (ONLY aerobics, and LOTS of aerobics) and never picks up any weights, or if they use weights, they WILL NOT use anything more than two or three pound handweights, there is something more there going on. I don't know how to define it or to describe it other than to say, there is "something" there that is not right.