The thing about looking at "old" people today who did no exercise is that it's nothing more than biased anecdote.
After all, many will have already died, so they're not considered! Yes, you may have an Auntie Madge who is 85, thin and healthy, but what about the long dead Auntie Flo and Auntie Edna???
And, by and large, they'd have had a healthier life in many ways, with less need for formal exercise. I.e. they'd probably not have driven early in their lives so would have walked more, to work, to shops, to cinemas, etc. Junk food wasn't so prevailent, so they'd probably have had a healthier diet of mostly unprocessed foods, and probably ate less generally anyway.
Today's "middle aged" are more likely to have had less walking in normal day activities, i.e. car to work, taxi for night out, etc., added to all the processed high fat/high sugar food, i.e. ready meals, tins, packets, etc., not to mention McDonalds, KFC, sugar laden special coffees and cakes, etc.
Completely different lifestyles.