ve'el-habakar ratz Avraham - Avraham ran to the cattle
Avraham could easily have sent one of his numerous servants to go to the stables and to select the animals for slaughter. After all, we know that he had a minimum of 318 male servants (14:14). He refrained from doing so, however, and went himself, and with accelerated gait, in order to honor his guests. All this in spite of the fact that he was both aged and weakened by the circumcision.
The Torah reports all this to round out the picture of Avraham as a generous person.