Why did Avshalom prepare all this?
Malbim: It was preparation for his rebellion. He conducted with royal etiquette to show his greatness.
Why does it say that he made men to run in front of him?
Rashi citing Sanhedrin 21b: He removed their spleens (Rashi 21b - through a potion, for the spleen weighs a person down) and the skin of the bottom of their feet, so they could run faster, and thorns would not hurt them 1 .
Radak (Melachim I, 1:5): They went in front of him, like the custom for armed foot soldiers to go in front of the king.
Would it not suffice that they wore shoes?! Chashukei Chemed (21b) - Adoniyahu wanted to show his grandeur, that people can run in front of him without tiring, and [barefoot] on thorns. Yad Ramah (21b) - some say that they did not have a bone in their feet.
