This leads me on to something else that occurred to me recently:
This was at the end of Caretaker, earlier in which Lieutenant Stadi gave Voyager's maximum velocity as warp 9.975.
If we use the TOS scale instead of the weird TNG one then this equates to 992.5c, which means that a 70kly distance could be covered in 70.5 years, so Janeway's estimate looks reasonably accurate.
The problem, though, is that maximum velocity does not determine the duration of such a long journey, average velocity does. The ship's average velocity isn't going to be anywhere near warp 9.975. I don't know if the Interpid-class's cruising speed was ever established, but if we go with DITL's guess of warp 8 (and that I think is quite generous) then the journey time would be closer to 137 years.
Even that, of course, is far too low: Voyager isn't continuously at high warp in the direction of the Alpha Quadrant, but dithering about exploring planets and getting into fights with the Garbage Pail Kids. Without a conveniently-placed wormhole or transwarp conduit there's no way they're getting back to Earth within a human lifespan.
Why, then, do so many of the characters - and the creative team - keep referring to the 75 year figure as if that's a realistic expectation?