They ate less sugar.
There was no "fast food".
There were no processed foods.
There were no artificial ingredients.

Those are all true but those aren't the only things that changed. There are also periods of time where nearly all those things were present but society was healthier and fitter. Namely, the 70's.

https://m.stacker.news/8205

Arguably, there were less of all of those things in the 70's, and I'm sympathetic to the view that those are key inputs to obesity and chronic health issues, but if we want to make change we can't rely on correlation alone.

Caloric supply also increased by 20% over those 40 years.

https://m.stacker.news/8207

These charts are from here.

I am also thinking how much of the work was more physically demanding vs the current demands of employment.

They weren't healthier. You're right that these unhealthy foods aren't good for health but people back then just suffered and died from different things.