I killed Reyes and got the mask back, returned it to the underground tomb. Yadda yadda the family is fine, then 5 months later Seth takes the mask, Marston returns as a zombie and then what? I have saved every town and rescued every missing person in the game, but nothing has happened. Am I missing something or is the game really just over?

That's it.  It just lets you carry on afterwards in case you want to play more.

Oh bummer. I had hoped for a little more with the ending to be honest. 

I know how you feel buddy, I started the exact same thread after that really underwhelming ending

Some towns get invaded again, other than that just roam around and do something. Maybe go to Fort Mercer or El Presidio, its always has loads of Zombies.