Obviously this raises the presidential speculation for Walker. It's hard to imagine any other office than the Oval Office that would take him from being Governor of Wisconsin. Tammy Baldwin's Senate Seat isn't up until 2018, the same year he'd be up for re-election if he did complete a second term. Or it could be as simple as Walker not wanting to lock himself in; a cabinet position with a victorious Republican in 2016 is also possible, as are a number of other possible career moves.
I think Walker is being completely honest; it depends on how much he accomplishes and how long it takes him to accomplish it.