I am retiring and going to write. How is the health of sedentary work at home folks?

Depends on exercise. Our bodies function the best with regular exercise. Many of the problems of our society in the us especially heart disease and obesity are related to not being active enough. One of the most important things a retiring person can do is develop an adequate exercise routine to replace the activity they probably did in their work a day life.
Not good. Writing stimulates the mind which is good. A sedentary lifestyle shortens life and leads to an increased rate of illness. You must remain mentally, physically, and socially active after retirement.