[Arcc]

Humans are tougher than you'd think. We're tenacious little buggers who don't want to die, and children in particular give up far less easily. The greatest threat in a survival situation is exposure, and if you can beat that and find ways to stay hydrated and keep your calories up, you can survive well enough for a very, very long time. Touma didn't look like he'd been playing Castaway though, so my guess, he'd been moving from place to place doing some odd jobs and relying on the kindness of others when he could.