Final answer:
The hardest thing for Nancy Mairs to survive without is memories of when she could walk.
Step-by-step explanation:
The author, Nancy Mairs, says that the hardest thing to survive without is memories of when she could walk. In her writings, Mairs often reflects on her experiences living with multiple sclerosis, which has gradually limited her mobility. She values her memories of being able to walk because they serve as a reminder of her past abilities and contribute to her sense of identity.