This was a thick book from the 1970s/1980s.
Man passes out in his home, wakes in a place where people live underground. He is asked to be guest at wedding. Above the ground it's barren.
He visits a school where children learn by going off into space attached by some kind of hose. At the end he is walking down road, surrounded by flowers viewing people getting older and almost returning to babies. He wakes.
I think he visited the cells of himself. What I mean by cells is the cells in his body. A world within himself. He is viewing the flowers and the people getting older as they walk down the path.