Really doubt it was inadequate fixing that is causing your fog. I have been hoarding this paper for a while, and it seems to be a crapshoot as to what is fogged and what isn't. Of course, I have no clue how many dank basements or steamy attics the stuff has been stored in. As far as the extended fixing times: two-baths of film strength fixer, 30 seconds each, has always been sufficient.
Your paper might be worthless for conventional processing, but it will still work very well with lith processing. If B&W is something you will be doing for a while, I would put it back in the freezer in hopes that someday you discover lith.