The last first day / Carrie Brown.

Enhanced descriptions from Syndetics:

From the author of The Rope Walk, here is the story of a woman's life in its twilight, as she looks back on a harrowing childhood and on the unaccountable love and happiness that emerged from it.
 
Ruth has always stood firmly beside her upstanding, brilliant husband, Peter, the legendary chief of New England's Derry School for boys. The childless couple has a unique, passionate bond that grew out of Ruth's arrival on Peter's family's doorstep as a young girl orphaned by tragedy. And though sometimes frustrated by her role as lifelong helpmate, Ruth is awed by her good fortune in her life with Peter. As the novel opens, we see the Derry School in all its glorious fall colors and witness the loosening of the aging Peter's grasp: he will soon have to retire, and Ruth is wondering what they will do in their old age, separated from the school into which they have poured everything, including their savings. The narrative takes us back through the years, revealing the explosive spark and joy between Ruth and Peter--undiminished now that they are in their seventies--and giving us a deeply felt portrait of a woman from a generation that quietly put individual dreams aside for the good of a partnership, and of the ongoing gift of the right man's love.