Congrats. Call-backs are a breeze. You meet with 6 or 8 partners, then usually 2-4 associates will take you out to lunch. It's more of a "does this person fit in" type of deal. You will be fine. At this point, according to NALP, a person has a 66% chance of getting an offer. However, I believe those stats are for 2L's, so I'm not sure how it works out for 1L's.
Regardless, congrats and knock em dead.