Exclusively Pumping mom going to a concert. Help!

My husband and I are going to a concert this Thursday. I have plenty of milk pumped and frozen for my son. We're gonna be gone from around 6 pm to midnight. When should I start thawing his milk? How long is it good for once it is thawed in the fridge? Also, my boobs. Any tips on how to not have painful engorgement while I'm gone that long? I plan on pumping right before I leave but I usually feel full after 2 1/2 - 3 hours.