Oh No..
Yes you should start immediately...
Dont panic, I am just kidding.
EAD is just a document that authorizes you to work. You are legal to stay in the US until a decision is made for your I-485 application even if you do not have any other valid non-immigrant visa (H1, L1, H4, L2, etc.,).
But make sure you do not work after your EAD is expired. That could get you into trouble from INS. Renew your EAD and then start working.
Consult an attorney if you need legal advice.
jericho02