I just finished watching this movie. Is there a follow up? Or was this a one off? How do the events of this movie fit into the main continuity?
The Short Answer:
No, there is no follow-up to the All Star Superman movie. The events of this movie do not fit in any of the DC Universe continuities. It is a unique event having unique parameters in the creation of the character "All-Star Superman".
The DC Animated feature film All-Star Superman was based on the comic book series All-Star Superman. As noted in Wikipedia:
All-Star Superman is a twelve-issue comic book series featuring Superman that ran from November 2005 to October 2008. The series was written by Grant Morrison, drawn by Frank Quitely, digitally inked by Jamie Grant and published by DC Comics. DC claimed that this series would "strip down the Man of Steel to his timeless, essential elements".
The series was the second to be launched in 2005 under DC's All-Star imprint, the first being All Star Batman and Robin the Boy Wonder. These series are attempts by DC to allow major comics creators a chance to tell stories showcasing these characters without being restricted by DC Universe continuity. [emphasis mine]
With that said, the All Star Superman was not a single Superman but an amalgam or archetype of the iconic hero across his entire heritage encompassing all of his previous incarnations. This was done to allow a well known writer to try and expound on the legend of Superman unrestricted. Grant Morrison used elements from across the history of the character to create a single story that embodied every Age of Superman. All-Star Superman is not the canon character of Superman, he is the paragon of the archetype.