The Iron Fist is not supposed to destroy the Hand. It is supposed to protect K'un Lun from the Hand.
From listening to Danny explain what he's been taught, it's clear that he always assumed the Hand was a fairy tale. As Iron Fist, his job was to guard K'un Lun from everyone, up to and including the Hand. He'd been told that the Hand was the sworn enemy of K'un Lun, and that his ultimate goal was to destroy them, but in the context of invaders. He was to destroy them if they came to K'un Lun.
When he left K'un Lun, though, he discovered the Hand was in New York City, which shocked him completely. He decides that the best way to protect K'un Lun from the Hand is to destroy them where he found them. He says as much to Davos when he comes to take Danny back. He convinces Davos that staying to fight the Hand is the right thing to do, but it's clear (especially by the end) that it wasn't his intended purpose, it was merely an unexpected opportunity.