Hi Tinevisce2016,
A 680 is a fantastic initial CAT score, so if you took this CAT in a realistic fashion, then I would say that you have a good chance to hit your score goal. Before we can discuss how best to proceed though, I have a few questions about how you took this CAT:
1) Did you take the ENTIRE CAT (including the Essay and IR sections)?
2) Did you take it at home?
3) Did you take it at the same time of day as when you plan to take your Official GMAT?
4) Did you do ANYTHING during this CAT that you won't be allowed to do on Test Day (pause the CAT, skip sections, take longer breaks, etc.)?
5) Since you studied back in 2015, had you taken this CAT before? Had you seen any of the questions BEFORE?
GMAT assassins aren't born, they're made,
Rich