OD - look up "Banting's Letter on Corpulence", dated 1864.
"I consulted an eminent surgeon, now long deceased —a kind personal friend,—who recommended increased bodily exertion before my ordinary daily labours began, and who thought rowing an excellent plan. I had the command of a good, heavy, safe boat, lived near the river, and adopted it for a couple of hours in the early morning. It is true I gained muscular vigour, but with it a prodigious appetite, which I was compelled to indulge, and consequently increased in weight, until my kind old friend advised me to forsake the exercise."
Exercise is great for all manner of reasons - but weight loss isn't one of them. Exercise makes you hungry, and there's a bunch of science about how and why, and the human mind is sneaky in the extreme about how it gets you to address any imbalance.
Being overweight is not a new thing (although the sheer numbers are - obesity in Banting's time was a rare metabolic disorder) - and up until the 70s, we knew what to do about it.