An active alcoholic in denial, as Charles Kennedy was, can fairly be described as unfit to run a national party. That's all that happened in that instance.

OTOH, I have known an active alcoholic who was able to maintain a prestigious career for decades, and to have lengthy and lucid conversations about current events which astonished his listeners. He was a writer who influenced millions of readers, although almost none of them knew his name. His memory for historical facts was amazing, as was his capacity to put away great quantities of hard liquor. He also played tennis almost every weekend. He was my father. He retired, had a lower-key second career, and died at the age of 72. Had he taken better care of himself he would surely have lived a lot longer. But no one who knew him would have described him as "unfit" -- for much of anything.

If we make physical fitness a criterion for public office I think we cross over into a territory that's crazy, ultra-controlling, and misguided.