There was so much to do that the sheer weight of it paralyzed Cain, rendering him unable to even decide where to start. So in the end he didn't start at all.
Instead, he wandered the Ancient City aimlessly for a while before deciding that there was nothing more that needed to be done here right away and started making plans to head back to the Southern Continent to grow his strength.
The Echoes were still tending the orchard that they had started, as well as protecting refugees from the Mythical Beast attacks, so he had a place to go and a purpose there.
He was certain that there were things he could do to help here, but everywhere he went that had a Guild presence just reminded him of Misha, and his mind would get sidetracked with thoughts of how to recover her, even though he knew he wasn't strong enough.