John Gill Commentary Jeremiah 12:5

"If thou hast run with the footmen, and they have wearied thee, then how canst thou contend with horses? and though in a land of peace thou art secure, yet how wilt thou do in the pride of the Jordan?" — Jeremiah 12:5 (ASV)

If you have run with the footmen, and they have wearied you ,
&c.] The Targum introduces the words thus, "this is the answer which was made to Jeremiah the prophet, concerning his question; you are a prophet, like to a man that runs with footmen, and is weary."

Then how can you contend with horses ?
or with men on horses: the sense is, either as Kimchi gives it, you are among men like yourself, and you are not able to find out their secrets and their designs against you (see (Jeremiah 11:18Jeremiah 11:19) ); how should you know my secrets in the government of the world, as to the prosperity of the wicked, and the afflictions of the righteous? Be silent, and do not trouble yourself about these things.

Or rather, as you have had a conflict with the men of Anathoth, and they have been too many for you; they have grieved and distressed you, and have made you weary of my work and service; and you have been ready to give out, and declare that you will be no longer concerned therein; what will you do, when you come to be exercised with greater and sorer trials, and shall have to do with the king of Judah and his court, with his princes and nobles, the sanhedrim at Jerusalem, and the priests and inhabitants thereof?

The Targum interprets the footmen of Nebuchadnezzar king of Babylon, and of the good things done to him; and the horses of the righteous fathers of the Jews, who run like horses to do good works, and of the much greater good reserved for them; but very improperly.

Much better might it be applied, as it is by some, to the Moabites, Ammonites, and Edomites, who gave the Jews much trouble; and therefore what would they do with the Chaldean army, consisting of a large cavalry, and which would come upon them like an impetuous stream, and overflow, as the swelling of Jordan, as follows?

and if in the land of peace, in which you trusted, they wearied you ;
if in his own native country, where he promised himself much peace, safety, and security, he met with that which ruffled and disturbed him:

then how will you do in the swelling of Jordan ?
when it overflowed its bank, (Joshua 3:15) and may denote the pride and haughtiness of the king and princes of Judea, and of the inhabitants of Jerusalem; and the difficulties that would attend the prophet's discharge of his duty among them; and the same thing is signified by this proverbial expression as the former.