If you already said you were going to go, you should probably go. It would be rude not to show up.
You don't have to socialise with everyone in the room, and you don't have to stay the allotted six hours. As long as you show up, the host will be happy.
To relieve some anxiety, always have a drink in your hand, even if it's just water. The more you eat and drink, the less you have to talk.