The idea of consistent travel times and warp speeds set sail with the Original Series and never looked back, nor should it be expected to. Things move at the speed of plot, whether you're in a series where it's supposed to be a long arduous journey across the galaxy, where you're "the only ship in the quadrant", on a day trip to the Galactic / Great Barrier or a five minute jaunt from the Neutral Zone back to Earth to fight the Borg.How could she lead a 4-5 year mission to the Lesser (Small?) Magellanic Cloud, some 199,000 light-years from Earth when it was going to take Voyager (with a sustainable cruise velocity of warp 9.975) 75 years to travel 70,000 light-years?
It can be assumed that the Federation's (+ Klingon + Ferengi + Cardassian, etc.) accumulated knowledge of thousands of years of spacefaring civilizations has catalogued a metric Spockton of beneficial spatial anomalies and regions of space where warp works faster/more efficiently than normally.
In TNG 'Force of Nature', we even saw that the use of conventional warp travel can damage the fabric of subspace in some regions more than others (something which was dropped by the time Voyager and DS9 came around, though that was said in peripheral materials to be the reason for Voyager's variable geometry nacelles, IIRC), which brings to mind the possibility that repeated use of warp drive in certain highly trafficked regions of the galaxy over centuries (like say from Earth to the Neutral Zone outposts or between Earth and Vulcan) could have experienced a less damaging version of this malleability of subspace that results in less "drag" and vastly increased travel times and more efficient fuel use; your "warp highways" if you will.
Speed also seems to increase when the interstellar medium / stellar density decreases somewhat, like on the outskirts of the galaxy or between the Milky Way and its satellite galaxies. Maybe going directly through the bulk of the Milky Way like Voyager did (because they had to scavenge for resources / travel shortcuts / allies along the way) cost them some extra travel time, but was the best option because they didn't have the fuel and stores stocked up for a journey up and outside of the denser parts of the galactic plane.
The alternative to this of course is STV with the Enterprise (and its BoP tag-along) reaching the Great Barrier towards the "center" of the galaxy in about ten minutes, which I contend is because "God" was a Cytherian (the giant floating head aliens from TNG 'The Nth Degree) criminal imprisoned on the Eden planet, which is why God needed a starship to escape his prison. He telepathically sent Sybok the means to boost a starship's warp efficiency (just as had happened with Barclay, albeit then through a probe, which may be their official way of making contact) and bring the ship to the center of the galaxy, and the Klingons cloaked and merged with the Enterprise's warp bubble to follow them.
Anyway, I'm all over the place here, but my point is essentially it's difficult for such a huge shared universe produced over decades to try and maintain rigidly consistent travel times and speeds and rules, and it was already slipping as soon as the first series started. If doing so places limits on the types of stories you want to tell, then the story should come first, and you can always come up with an explanation later, or don't. YMMV.