The ending of the story is a little bit ambiguous, and it isn't what we would typically call happy or sad. It is a victory for Phoenix insofar as she proves her strength and determination to carry on in the face of rough terrain, physical hardships, and the haters she meets along the way. Medicine in hand, she's on her way home.
Even though reaching the doctor's office is a victory, the story doesn't go out on a slam-dunk-win kind of feel. We don't know if Phoenix's grandson will ever actually get better. What we do know is this: Phoenix "lifted her free hand, gave a little nod, turned around, and walked out of the doctor's office. Then her slow step began on the stairs, going down" (100). With the same methodical determination we witness in Phoenix's journey to the city, she starts the long journey back.
Phoenix will keep on keeping on, which makes the image of the windmill in the second-to-last paragraph a fitting one for the end—to dig into why, swing by the "Symbols" section.