Relax and take a breath. If you order the AppleCare when you order the iPad, it will be applied at the same time. But from your post, it looks like you ordered the AppleCare well after the iPad. In that case there will be a registration number in the packet that you would use to activate and match it to your iPad. If you are not home to take delivery AND you are not able to activated it before your warranty expires, you may well get an error message when you try to activate it. Don't panic, call the contact number with your web order number available. They will be able to activate the AppleCare if you can show proof that it was ordered before the warranty expired.
The expiration date of the AppleCare will be the same, regardless of when you purchase it, the same time you purchased the iPad or the day before the warranty expires.