The actor, who has agreed to reprise his role as wizard Gandalf for the two-part J.R.R. Tolkien adaptation, revealed why he has only just put pen to paper on a contract.
"The deciding negotiation was not about money but about dates," McKellen wrote on his website. "Gandalf is needed on set over the next 18 months but with sizeable breaks when I can work on other projects. My worry that I could not easily escape from Middle Earth was lifted."
McKellen stated that the ongoing talks led him to consider whether or not he should take on the role again.
"As my agent continued to negotiate with Warner Bros, I kept wondering was Gandalf what I most wanted to do, more than a new play for instance or indeed a new part?" he added. "Sequels aren't necessarily as rewarding to act in as their originals.
"Could I let Gandalf go? Would anyone else care if I did? Elsewhere, does anyone care that Michael Gambon was not the first to play Dumbledore?"
However, McKellen said that he believes Martin Freeman to be the "perfect" choice for Bilbo Baggins and praised Peter Jackson, Fran Walsh and Philippa Boyens's script for The Hobbit: Part 1.
"It was true to the style, the fun and the romance of the trilogy," he commented.
The Hobbit will begin production on February 21.