WoW Free Trial Download?

So I just signed up for a 10 day trial for World of Warcraft, and the download is 9.7gb. It has 3 stages, setup, available, and playable, it's been downloading for 5 hours now, and its still got 3gb left. However, it does have the play button next to the download bar. Can i just hit play, and play the game, or do i have to wait for the download to be completely done? Thanks