From this: "I have not been able to walk for enjoyment or pleasure or exercise in 41 years" - I'd guess the usual way basketball ruins bodies - it's probably fucked up his hips and knees.
Edit: From one of the links:
“I have had 38 orthopedic operations,” Walton told Bicycling. “Both my ankles are fused. I have an artificial knee. I have a new spine. I have broken countless bones. Every bone in both hands. Every bone in both feet; both legs. I have broken my back. I’ve had countless facial and cranial fractures. I spent my life falling down, getting hacked and fouled and getting in fights.”
If you enjoy something enough, you keep doing it in spite of some setbacks or pain.
Over 25 years I've cracked ribs a few times and dislocated a shoulder once from not being a good enough motorcycle rider. I am still riding motorcycles.
If you enjoy something enough _and_ are making a shitload of money doing it, I'd guess some people's injury list would get quite horrifying before they chose to be "done".
Edit: From one of the links:
“I have had 38 orthopedic operations,” Walton told Bicycling. “Both my ankles are fused. I have an artificial knee. I have a new spine. I have broken countless bones. Every bone in both hands. Every bone in both feet; both legs. I have broken my back. I’ve had countless facial and cranial fractures. I spent my life falling down, getting hacked and fouled and getting in fights.”