Maybe you got older... I know my body changed at 44 or so, and I actually have to pay attention to what I eat and exercise. That's about the same time I quit smoking. I'm 10 lbs up from smoking today, but that's including winter fat. (I'm actually at 210 naked.) I'm aiming for 196 this year, and I will make it.
The calories from food (fat) is not pooped out, it's exhaled out. So if you can get yourself moving and breathing hard, you can start burning calories. Doesn't really matter what you are doing, you just gotta start to get winded doing something. The faster you breath from exercise, the faster you burn it.
An example fat molecule:
It comes out as water vapor and carbon dioxide.
I did a somewhat scientific study on this by eating the exact same thing, drinking the exact same thing, and estimating the number of breathes while exercising for a couple weeks. I came up with a result that was I should have lost 1500 kilocalories in a 20-mile bike ride off-road (about 1/2 lb a day). I did.
Keeping that up? A little difficult, but it can be done (but not every day.)
Diet was simple.
Breakfast: 2 eggs, 2 bacon, fried potatoes.
Lunch: 1/2 grilled ham and cheese sandwich
Dinner: Chicken (3/4 thigh), veggies (qty varies) and 1/4 cup rice (dry measurement).
Snacks allowed (protein bars) only during bike trips.
Boring, yes, but I confirmed the study for myself. References available on request.