In Jeremiah 12:5, we read the words of God to Jeremiah: “If you have raced with men on foot, and they have wearied you, how will you compete with horses? And if in a safe land you are so trusting, what will you do in the thicket of the Jordan?”
Philip Ryken explains these verses as follows:
The point is that Jeremiah hadn’t seen anything yet. The troubles he was having in Anothah were nothing compared with the troubles he would later have in Jerusalem, Babylon, or Egypt. Things were bad but not the worst. If Jeremiah thought he had trouble today, he needed to wait until tomorrow. Anyone who gets discouraged downtrodden and defeated over many little things will never fulfill his divine calling. If even little disappointments tempt Jeremiah to leave his calling how will he cope with real persecution?
God has great things in store for Jeremiah but he would never achieve that unless he was willing to persevere in the little things. He had to be willing to race with men before he could compete with horses. The same is true for every Christian. If you complain about the simple things God has already asked you to do, then you lack the spiritual strength to do what he want you to do next. If your troubles keep you from doing the Lord’s work now, you will never have to strength to do it later. If you want to do something great for God and you must begin by doing the little things for God, and the only way to do the little things for God is to do them by the strength of the Holy Spirit.
Philip Ryken, Jeremiah & Lamentations, “How Can You Run With Horses?” 222.