Can I walk away even though I just promised not to do exactly that?
Is there a reason you want to walk away? Did something happen?
I am looking for a way to get out of it at the right time.
You should be honest if you don't want to do something. They might need time to fund someone else to fill your position.
I just don't want to be held down by anything.
Wouldn't it be a great feeling to have your friends know they could count on you when you say you'll do something? It can be annoying to have people asking things of you, but you gave them your word and should stick by that rather than mislead them.