I'm on the opposite end as j3nn. I've never known a thin woman who ate much more than 1500-1600 cal/day. Nor one who ate three full meals a day.
Also, from Atkins to Primal, I've read (here) and heard in real life over and over that just going low carb with no calorie restriction works for about 40 pounds and then levels off. That 40 pound number (actually 43) was also my personal loss if I didn't restrict calories and only restricted carbs.
Exercise is good for many things, and not exercising will probably make life tougher with each decade that passes. But it pretty much sucks for burning calories. Building muscle will make you more of a calorie burner (because muscle is more metabolically active), but lifting weights doesn't burn a lot of calories. There are a few exceptions. Scuba burns a lot of calories.
The easiest way for me to adhere to 1200 calories per day is to build it around the meal I like best. For me that's dinner/supper. Maintenance is the same - build the 1500 cal around my favorite meal. Also, remembering that it's calorie deficit over time. So, plan for social events. Thin people occasionally overeat; most likely they compensate for a couple of days afterward. There was also a PBS show many years ago that claimed that naturally thin people showed a slight metabolic uptick after overeating while people who gained easily didn't. It ain't fair, it just is.
Lastly, once you find something that works for you, whether it's calorie restriction, carb restriction, or hey, even telling society that 140 pounds at five feet tall is just fine, put your fingers in your ears and listen to yourself. What works for me might not work for anyone else, and what works for you is all you need to know.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine