As I stated in previous posts concerning these mass account sales on the dark web, all is not lost and you should not panic. The advice in this article is sound. If you did not change your password on LinkedIn in 2012 after the initial breach, you should do so now. Use a complex password and consider using a good password manager like 1Password to help you keep track.
Also, LinkedIn has implemented a good two-factor authentication solution. You should take the time to set that option up to ensure that if your username and password are compromised, you have another good layer of protection in place.