The approchyra states that while high warp velocities are attainable they require the warp core shut down and cooling after heavy usage. For example in real world air forces, jet's today require several hours of upkeep for every hour it spends flying. The starships require several hours of cooling and maintenance for every hour it spends at high warp velocity. In the TNG episode Legacy they did just that PICARD: Engine status. DATA: Stable. But we have maintained warp factor nine for longer than is recommended, sir. That's why even at maximum warp for Voyager it should traverse the 70,000 lightyear journey in a decade or so (assuming having to stop for fuel/spare parts), not 75 years.......While it can attain maximum warp speed for about 12-14 hours consecutively, every hour at maximum warp afterwards requires about many hours of cool down, maintenance and ship inspection. Transwarp works on different principles of "Star trek physics". In an episode of Voyager they stole a transwarp conduit to cut their journey time, presumably. Even if the merger of technology didn't achieve "True" transwarp it did shave considerable distance off their journey (though it's never specified how long it was operating) presumably by augmenting their ships warp field with subspace altering tweeks...Thus allowing the ship to either exceed it's highest warp velocity or operate as such for a much longer duration.
