This is the 4642nd most frequent Spanish word.
cansó
"Got tired" or "wearied."
In this sentence, 'cansó' means 'tired', referring to the action taken by the subject (hard work) that caused fatigue in another entity (him). It is the past tense of the verb 'cansar' in the third person singular.
El trabajo arduo lo cansó después de tantas horas.
The hard work tired him out after so many hours.
Here, 'cansó' is used reflexively as 'se cansó', indicating that the subject became tired through their own action (running).
Se cansó rápidamente al correr esa distancia.
He got tired quickly when running that distance.
In this context, 'cansó' is used to describe how the subject (he) caused fatigue or boredom in others, people around during the event described.