The updated Survival Mode is now undergoing its Beta stage and is available for play on Steam. The update also features gameplay optimizations and the support of the upcoming DLC, Far Harbor. The new Survival Mode has features such as no fast travel, saving only when you sleep, increased lethality, diseases, fatigue, danger and more.
As of now, the characters from the Beta don't have a backward compatibility with the previous versions of the game, so don't try using Beta characters' save games outside of the Beta.
For more detailed description of the patch notes visit this link. If you want to read the instructions on how to participate in the Beta, scroll down to the bottom of the page.