Exercise can be a means of purging for bulemics.
To turn this on its head then, perhaps the answer to the rhetorical question "what's your excuse" could therefore be "I don't have a potentially fatal eating disorder."
Edit: I initially posted this thinking it was just an interesting hypothetical to consider, but
this suggests she is, in fact, not as healthy as her image suggests, and justifies much of the debate about the message she's sending and the place it comes from.