It's a tricky one. I became obese while working just by sitting still at my desk all day, as well as a round commute of 3 hours sat down on the train each day. I actually ate quite healthily, but I just wasn't burning off what I was eating, and it crept up on me over the years.
When I retired, my priority was to lose some weight as well as getting fit and staying fit, and I'm still heading in the right direction.
In terms of fast food, I think the biggest current issue is delivery services, so some people don't even get the exercise walking to the takeaway before stuffing themselves with crap.