In my experience, sitting leads to a desire to eat. I inevitably start thinking about what I am going to eat next. For me, going for walks is a way to avoid that. Going for a walk is also a good thing for reasons having nothing to do with weight (or 'fitness'--I'm talking about a nice, relaxed stroll, not 'exercise'). If you aren't doing anything in particular and you start to think about 'something to eat', if possible go for a walk instead.