Just simple advances have contributed greatly to our increased longevity. The availability of clean drinking water distribution and sanitary waste disposal systems made a huge difference. For example: London circa 1850 with the contamination of the local water wells by the raw sewage from the Thames river causing Cholera outbreaks. Not to mention the stench. And for all the whining about exhaust emissions from cars, think about the methane produced by 275 million horses on the streets of America if there were no cars not to mention the disposal of the road apples they leave behind!
Your scenario is disgusting to contemplate. But how different a country we would have if such as you speak existed. In order to create so many "road apples" the horse population would consume vastly more food than his human master. Our economy would ever have matured toward manufacturing if all available land was employed growing horse fuel.