What did Tom learn in prison that he is able to convey to Ma when she enquires about the length of the journey ahead of them?

From the text:

"I dunno," he said. "Two weeks, maybe ten days if we got luck. Look, Ma, stop your worryin'. I'm a-gonna tell you somepin about bein' in the pen. You can't go thinkin' when you're gonna be out. You'd go nuts. You got to think about that day, an' then the nex' day, about the ball game Sat'dy. That's what you got to do. Ol' timers does that. A new young fella gets buttin' his head on the cell door. He's thinkin' how long it's gonna be. Whyn't you do that? Jus' take ever' day."