ASK what the OG is when you pick it up. Sounds like it will basically be a no-chill cube full of sterile wort, do NOT risk opening it up and contaminating it to make a plan after doing a gravity reading. The brewery should be able to tell you exactly what you have. While there, ask them about getting some house yeast. Lots of breweries will gladly give you a few cups of super concentrated yeast if you ask nicely. If it it IS cubed good wort, you are in no huge rush. Spending a couple days making a starter should not hurt you at all.