Years ago I jumped from 200 pounds to 250 (I am 6ft) after I moved to Knoxville. I was in my 30's, and I just took my first job after college (graduated from ETSU) as an accountant. I was eating a bad diet of large lunches (I think I ate at every resturant near Lovell Road and Turkey Creek) and a lack of exercise (sitting at a desk all day can be murder on a body). As a former Marine I was disgusted with myself. I cut out sodas, stopped eating out for lunch, and limited my red meat intake while running/walking 3 times a week. Also I limited myself to 1500 to 1800 calories a day. I started to drop weight like crazy. Luckily I love foods like sushi, any kind of fish, and chicken. Within 5 months I dropped 77 pounds, and I was down to 173. I went from a 36 to 38 pants size to a 32.
So in the end you are correct. It just takes discipline. I do question fad diets like this, because you need truly a lifelong lifestyle change to keep the weight off. Once you go off a fad diet the weight will come back.
I'm going to be 42 next month, and my weight now stays between 182 and 175. I now drink a whole lot more (craft beer is my obsession which usually is high in calories), but I still avoid high calorie meals during the week. If I start to get close to 182 I become more 'nazi' about my calories. I also still run a couple of miles 3 times a week (my motto is 'I run so I can drink beer'). So for me it is really about the calories more than anything.
One warning the older you get the harder it will become. I use to be able to drop 5 pounds in a couple of weeks, but now it can take me up to a month. You just got to stay on top of it.