Ok here is the story as I remember it: Set in a alternate universe where there are still Kings and Lords in this story the second son of a King is sent across the ocean or sea and joins that Kings army and sleeps in the barracks with the other soldiers the Princess takes a liking to him but a war starts up with the richer nobles who want to rule the land by killing the King so he joins with the others and travels by wagons and horseback to defeat the nobles due to distance between lands it a long time to return. He then has to go back and fight his brother who tried to kill his father to take over early.
It's not "The Third Son" by G.Younger that was my first check
Hopefully many minds are better the one.