The technical manuals aren't cannon... but we do have few pieces of info to give us a better understanding of Starfleet's Warp capabilities in the 24th century.
I take issue with the premise it would have taken Voyager over 70 years to get back. Mainly because according to canon data, Warp 9.9 = 4 billion miles per second, and that would result in 21 473 times speed of light. Even Harry Kim stated in the early seasons 'traveling at relative velocity of 2 billion miles per second' - or 10 736 times speed of light - when discussing a plan to beam Chakotay out of a Kazon ship. Even at that velocity, it would have taken Voyager 7 or 7.5 years to get back - the writers could have gone with that.
Voyagers maximum sustainable cruise velocity was stated several times to be Warp 9.975. Past Warp 9.9, each increment results in exponential increase in velocity, which would mean that Warp 9.975 would take Voyager about a week to traverse 75 000 lightyears.
And no, it's not far fetched. Most Federation ships were never seen achieving Warp 9.9 (or even 9.5) sustainably (and by that I mean as a cruising velocity) before Voyager in the first place, and the damage Voyager received in it's initial pull into the DQ was extensive (probably preventing them from achieving those speeds and forced them to use slower velocities like Nebula class ships of the time, while their 'maximum sustainable' speed was reduced to say 9.5 and that would have still trained the engines - as Tuvok mentioned in Threshold episode that Paris exceeded their maximum velocity, which was mentioned to be 9.9).
I know Threshold isn't canon, but it's giving us better insight that Voyager's writers never really showed or pushed the ship past 9.9 in the first place, plus, in that same episode, when Voyager was chasing mutated Tom who kidnapped Janeway and initiated modified Warp engines to reach/break the Warp threshold, Voyager was unable to sustain warp 9.9 as is evident from the dialogue:
"KIM: They're approaching warp nine point nine.
CHAKOTAY: Increase speed to match.
COMPUTER: Warning. Nearing maximum warp velocity. Structural collapse is imminent. "
Given the damage sustained from the pull to the DQ, I would surmise that Voyager's engines and hull took a beating that would have resulted in drastic reduction in Warp speeds that the crew was unable to repair on the go without specialized access to a drydock - either that or access to an uninhabited star system that they could have used to harvest solar energy for the replicators to generate what they needed - or again, an uninhabited system with tons of asteroids that have simple raw matter that could have been converted into energy and back into something they could use to repair the hull - though Voyager hadn't made such pit stops... except in Season 7 to do a major overhaul, but nothing was mentioned about them harvesting the sun for power to generate energy for the replicators (even though its well within their capabilities).
I suspect that this is how Voyager was able to replenish their shuttles and torpedoes eventually... by making small pit stops in uninhabitated star systems and using say solar power for replication. And of course trading with other species.
IF the writers had more sense they would have showed us how Voyager was really self-sufficient and could have initiated trading for the sake of first contact with other races (and explain more torpedoes - which could easily be manufactured if you have enough antimatter - and I suspect that they can generate more via several methods on the go).
And it also ties into the premise that when receiving a message from Starfleet (Message in a bottle), Admiral Hayes mentioned that 2 deep space ships were redirected to Voyager and would hopefully meet with them in 5 to 6 years.
Voyager was at that point 60 000 Lightyears away, and depending on where those ships were in the galaxy (possibly exploring the Beta Quadrant), they would still need to cross tens of thousands of LY's in that time frame... which would indicate a speed of at least 2 billion miles per second (or lower than Warp 9.9).
Possibly Warp 9.5, or 9.6 (again the closer you get to Warp 9.9, speeds experience much higher increases, but past 9.9 each increment results in doubling of speeds).
Most SF ships we usually see on the shows are the Defiant and Enterprise... they tend to 'cruise' on low Warp speeds... and only use Warp 9 and above when in a hurry.
This indicates to me that SF has dedicated deep space ships capable of traveling at above Warp 9 for long periods of time and traversing about 10 000 ly's in 1 year (at least).
That is until they developed the Intrepid class with 'sustainable cruise velocity of Warp 9.975... but, as we know, Voyager was thrown into the DQ and suffered massive damage in the process.
The writers never paid too much attention to that detail, but coincidentally it DOES add up... and is far more credible than the premise that Warp is ridiculously slow (which really doesn't make any sense for a space faring culture that's evolves at faster than exponential level - or is at least supposed to).
Also, there's the USS Prometheus from Voyager's Season 4.
That ship was able to sustain Warp 9.9 as a cruising velocity it would seem, or at the very least, didn't have issues sustaining it... and other Starfleet Ships were able to catch up to it. I suspect the Nebula class ships got a nice upgrade to their engines during the Dominion War, allowing them to reach Warp 9.9 sustainably... and above for shorter periods.
I suspect that older designs might be having issues reaching past 9.9, but it wouldn't be a stretch to think that during and by the end of Dominion War, most Starfleet ships would have been capable of sustaining Warp 9.9 or slightly above as a cruising velocity.
Any new designs like the Intrepid class would likely be able to have a sustainable cruise velocity of Warp 9.975... or 9.98... with 9.99 being 'temporarily sustained'.
A sustainable cruise velocity means an engine can sustain a said speed for indefinite periods of time without duress (or in other words, the speeds at which a ship is meant to run at most times).
That is if you want to reconcile in-universe explanations.
Outside the universe...we know the writers didn't pay too much attention to these things, and unfortunately didn't understand 'faster than exponential development' when you factor in a space faring collection of civilizations like the Federation and the technologies they had to work with.
Voyager's warp issue is easy to reconcile with the premise that Warp is actually much faster than what we are led to think and that Voyager specifically just ended up with too great damage which they were unable to repair in the DQ... its just that most SF ships (sans deep exploratory ones) until Voyager didn't fare that well in sustaining a cruise velocity of 2 billion miles per second (and it would make sense if you wanted to go from one end of the Federation to another in a relatively brief amount of time without having to wait a decade... otherwise, prospective cadets attending SF academy from other Federation worlds for example would likely be unable to attend classes directly on Earth, but rather in space on route... which is doable from what we saw, but overall, most people just go to the Academy... and that doesn't track with the idea that Warp is slow).