Why did Avraham run to the bulls?
Seforno: Refer to 18:5:3:3*.
Midrash (Pirkei d'R. Eliezer), Hadar Zekenim: When Avraham approached the bull to Shecht it, it fled, and Avraham gave chase. The bull actually led him to the Me'aras ha'Machpelah. 1
Ramban (on Pasuk 8): So great was Avraham's desire to perform kindness that, in spite of the three hundred and eighteen able-bodied members of his household, his advanced age and his current physical condition, he opted to go personally to Sarah's tent to encourage her to quickly prepare bread, and then ran to the meadow to choose a juicy bull for his guests!
In fact, that is how Avraham discovered the Me'aras ha'Machpeilah. (See also Ba'al ha'Turim).