The devotion you shared points to something true and worth holding onto. God does not send His soldiers into battle untrained. When He calls someone, He also prepares that person to stand in the calling.
Scripture reminds us to have confidence in the Lord, which is to trust His lovingkindness. Yet that confidence was never meant to make us passive. We are to cast everything upon Him, and at the same time to work ourselves, to do the things He has commanded and to continue in them with perseverance until our last breath. The training you speak of is this very life of faithfulness, day by day, not turning back.
That training comes especially through prayer. The one who prays as he ought, and who learns to restrain the desires of the flesh, becomes light and winged in soul. Such prayer lifts the mind above earthly things, quenches harmful passions, humbles the heart when it would be lifted up, and makes a person strong against the enemy. Nothing is mightier than a person who prays sincerely. If you find your body weak for much fasting, you can still keep watch in prayer and turn away from excess, and even this is a real part of being made ready.
Remember also how God calls. He did not choose many who were wise or mighty according to the flesh, but He chose what the world counts as foolish and weak, so that His power might be shown and His wisdom might be proved. His strength is made perfect in weakness. So do not measure your readiness by worldly ability. Your calling itself is a gift of His love, and from that love comes sanctification and every blessing. You are called to be His, beloved by Him, and that is where your preparation begins.
It is right to cast your cares upon the Lord, for He does care for you. And it is true that if God is for you, no one can finally stand against you. That does not mean there will be no struggle, but it means the struggle is in His hands and He will keep you as you abide in Him.
May the Lord train you in humility and courage, in prayer and in steady obedience, and keep you faithful in the work He has given you. May He lift your cares from you as you place them on Him, and teach you to walk each day trusting His power rather than your own.