Did he run because he knew they were angels? Or did he run to exhibit hospitality to strangers in the desert?
From the commentary of the Malbim:
וכאשר ראה אותם עוברים בצד האהל כרוצים לעבור לדרכם ,רץ לקראתם לעכבם שיסורו אל ביתו
When he (Avrohom) saw that they were passing by the tent as if they wished to resume their journey, he ran to prevent this so that they should turn into his dwelling.
It seems that he was acting out of hospitality.
As Sforno writes,
he began running from the entrance of his tent. When a person is observed to perform any task with speed, with keenness, this reflects that he considers what he is about to do as important. We have another example of such conduct in Exodus 34,8 וימהר משה ויקוד ארצה וישתחו, “Moses hastened to bow down and to prostrate himself.”
as to the question of how Avraham saw them, the Radak writes,
Our sages (Bereshit Rabbah 50,2) say that Avraham was so familiar with the sight of angels that their sudden appearance did not cause him to become disoriented, frightened and he perceived them essentially as אנשים, men.
Based on Rashi, the essential companion for any study of the Torah, Abraham thought they were strangers and did not know that they were Angels:
The men rose from there and looked out upon Sedom, and Avraham was walking with them to send them on their way.
TO ESCORT THEM - for he believed that they were travelers.