Albert Barnes Commentary 1 Kings 1:5

"Then Adonijah the son of Haggith exalted himself, saying, I will be king: and he prepared him chariots and horsemen, and fifty men to run before him." — 1 Kings 1:5 (ASV)

The narrative concerning Abishag the Shunammite is introduced as necessary for a proper understanding of Adonijah’s later history (1 Kings 2:13–25). As it stands, the account also considerably heightens the picture of the king’s weak and helpless condition, which Adonijah was not ashamed to exploit for his own aggrandizement.

Adonijah was born while David reigned at Hebron and was therefore now between thirty-three and forty years of age. He was David’s fourth son but had likely become the eldest after the death of his three older brothers. He claimed the crown by right of primogeniture (1 Kings 2:15) and secretly announced to his supporters his intention of assuming the sovereignty (compare to 1 Kings 1:10). It was well known to him, and perhaps to the people generally, that David intended to make Solomon his successor (1 Kings 1:13).

To run before him—that is, he assumed the same royal-like status as Absalom had done when he planned his rebellion (2 Samuel 15:1).