He has a severe heart condition. If he were not super rich and famous he would have died over a decade ago. It is not unusual that men who must sustain a very high calorie diet for a long time in order to build and maintain his kind of muscle mass, will end up obese in their old age and often do not have as long of a life expectancy.
Mesomorphs like Arnold generally have a life expectancy of 60–70 years. Kind of like a big dog might live 7 or 8 years compared to a regular sized dog that might live to be 12 or 13 years old.
Fact is Arnold should already be gone. And like most of our heros who stay too long, he has soiled his prestige by selling out to the corrupt establishment and the longer you stick around after doing that, the more inevitable it becomes that you will be exposed for the sell out has-been hack that you are.
In short, it has nothing to do with Kelly McGillis.