The work that the chubby kid had to do was help Lith in defeating the strong monarchs of other kingdoms in this world.
He didn't directly say this as he knew that the kid was smart enough to know that he was being used. Hence, the message was indirect and Lith eventually embarked on a mission to defeat all the monarchs in this world.
Defeating the monarchs of this world took roughly six months. Travelling was a hassle here and magic didn't work, so one had to do it on foot.
There was a lot of running that took place and Lith had to do it while having the chubby kid on his back. It was annoying, but this dude's presence turned out to be fruitful as he did help defeat the stronger opponents.
Lith was strong to the point of winning against just about anybody in this world, but there were always exceptions and those were thankfully taken care of by the chubby kid unknowingly.
