Navigable is relative when it comes to wormholes since the singularities simultaneously exist in different locations. If the singularity (speaking from a layman's point of view) is gravitationally infinite then a traveler would likely get ripped to shreds as soon as they crossed the event horizon. If gravitational forces are not at play and you could "enter" the singularity safely you would emerge at the other singularity (regardless of time or distance) unscathed instantaneously, theoretically. Reasonably, wormhole travel would not be like it is depicted in Star Trek, Contact, or any other sci-fi series using the plot device to move people from point A to point B.