Hi Noach,

Very interesting question.

You are assuming that the we should look in modern medical books to understand what R' Yochanan meant. I cannot say if you are right or wrong, but I it could possibly be that R' Yochanan is just reflecting his 'Hashkafa' or his thoughts of what is important in one's life. I gathered some explanations that were given by different Mefharshim.

It seems like R' Yochanan, besides being a great Torah scholar, had a lot of different virtues and skills. These are used in different ways to explain what R' Yochanan meant.

A. He was extremely beautiful (Bava Metziah 84a) and well dressed (Shabbas 113a). The Alter of Kelm says, that this might hint that R' Yochanan saw importance in the way a person or at least a Rabbi should appear. This explanation offers more of a thought that thinking of one's health is important, rather than trying to find the medical reasoning.

B. He had ten sons who died within his lifetime (Brachos 5b) and some say he lived for very long years (Raavad in Sefer ha'Kabala). It is said over from R' Chaim Kanyevsky, that this might explain R' Yochanan worry to be left without sons who would help him in his older years. I can add that this explanation too is more on the psychological side, and not that R' Yochanan meant to give medical advice. BTW, in Gittin (70a) we find that he actually took action and used remedies that kept him strong and young.

C. In his younger years he lived in Tzipori. In Eiruvin (56a) the Gemara says that a town that has a lot of hills and vallies, the people of that town age early. Tzipori is certainly a town like that as we can obviously see. The Ahavas Eitan offers that this is the reason R' Yochanan was concerned of his old age, and was leaning on his students when going up and down the hills and vallies.

I hope I helped a bit,

Kol Tuv,

Aharon Steiner