From my experience: I didn't have a sedentary life, well at least lets just say I wasn't sitting at a computer all day. I was always out and about. My extra weight was from a mix of overconsumption and poor food choices.

I noticed there were unforseen changes in behaviour due to diet that I still find strange. Things like quitting pop. I used to grab McDonald's food more often, but since quitting pop I avoid the place. Sometimes I still grab a burger from there, like 5 times or so over the last 14 years, but I'll never get a pop and I don't have a desire to go there and still avoid it with ease.

I still think diet changes are the single most important thing, but I just don't think I could have gotten there without exercise.