I’ve learned since starting running that people really overestimate the calories you burn - I’ve heard that a mile, whether you run it, walk it, or sprint it, is about a hundred calories. That means that in a long-ass day, the walking part burned 450 calories, which if there’s something tasty in the nurse’s lounge, well, there you go.
I mean, of course nurses burn calories in other ways - just standing burns more than sitting, for example, and there’s lifting and all sorts of other physical activity. But it does point out how diet can very, very easily overwhelm exercise.