" the guarantee of the Rambam, who concluded his treatise on health with the promise: “Whoever lives in accordance with the directions I have set forth, has my assurance that he will never be sick until he grows very old and dies; he will not be in need of a doctor, and will possess a strong constitution and enjoy good health as long as he lives " What are these directions the Rambam set forth to attain good health?

The assurance you quoted appears at the end of a brief resume of the Rambam's ideas about healthy living, which include healthy diet, sufficient exercize, moderation in all habits including sexual activity, etc. The following weblink has this chapter in English.