My sister and her fiancee are both personal trainers/fitness instructors. Some observations they've shared with me:
1) 200 or 300 calories an hour, um, that's not really exercise, in the sense that you wanna define "exercise" in the context of weight loss. That's a rate of exercise that you'd recommend for someone who is very, very, overweight, the sort where you're worried that having them hit a heart rate > 120 might not be safe. This is not the majority of people 50 or 60 years old.
200-300 calories an hour, um, that's not even breaking a sweat for a lot of people. Call me old-fashioned, but I think if I'm not breaking a sweat, I don't really call it exercise.
2) The math on this shows nothing surprising. Estimate 2000 calories net gain/loss over a timeframe of weeks per pound of fat gained/lost.
So you wanna call burning 200 calories your "workout"? OK, no problem. But with no changes to diet, and assuming someone is eating basically their "revenue neutral", no loss, no gain, amount of calories per day, well, if you exercise 5X a week, you're going to lose a pound every 2 weeks.
So a really anemic, unsatisfying rate of weight loss is what you get if you do a teeny, tiny, bit of exercise that barely breaks a sweat and don't change your eating habits. Film at 11.
3) My sister has a client that I met, once, then didn't see for a year or 2. She had lost at least 100 lbs. So I asked my sister "aren't there some people that are still fat, even if they exercise and eat properly?" and she told me, not that she'd seen. She told me the number of clients she'd had who had decent eating habits in the 2000-3000 cal/day range, who exercise 3X/week (that is, vigorous, 500 cal/hour, think running intensity level) for an hour or more at a time, who started out overweight and stayed just as overweight was exactly zero. She hasn't even ever _heard of_ another fitness instructor who had someone like that.
4) One problem my sister identified in a lot of clients, especially people who are hitting the gym for the first time in their life in their 30s or later and never played any sports, is they don't want to do a level of exercise intensity that they find "uncomfortable". Anybody who's ever done anything even remotely athletic in their life will know the truth of "no pain, no gain", but there are some people who don't want to endure the "pain" part of that equation. Those people have a lot of trouble losing weight, and often feel that "it just never works for me", and only when they get one-on-one training with a fitness instructor (or go to the gym with a fit friend) that anybody has ever pointed out "you're doing it wrong".
And, ok, profoundly unscientific, but at least in my own personal experience, I'm amazed at how much exercise I can do, and how much I can improve my fitness level (by whatever measure you like) and have zero or next-to-zero effect on my waistline, and by how significant effects on my waist that I _do_ see, with what seems like a pretty small change in diet. Like, as in, switch from coke to coke zero, and changing morning "coffee plus donut" to "just coffee".