They clearly didn't think it out but the implication is the mimic Voyager had to get enhanced warp drive from near the area of space "Demon" is in.
When they set out for Earth, they had to pass through the nebula ("One") and they had to have encountered the area Arturis was found in ("Hope and Fear"), the nameless unseen trading colony. From there, Voyager had to warp off to the side to the Dauntless and then there was the test flight and then the rescue mission. Mimic Voyager would have set out ahead, passing through those 300ly Voyager skipped over after using the slipstream after returning from the rescue mission that was going back to Borg space (to Species 116's homeworld it seemed). The Void starts at some point after that.
Logically, either they encountered another race at the trading colony who had the enhanced warp drive (EWD) technology/expertise or they picked that up in the 300ly since then. We see nothing unusual at the warp core, but among Neelix's unaffected supplies were: particle accelerators, trilithium ore... and a keg of Hazari ale. That might be a tip... trilithium. Trilithium + accelerated alcohol particles= enhanced warp drive.

Or maybe the Silverblood mix combined with trilithium is the enhanced warp drive. Dunno how dichromates, deuterium, hydrogen sulfate would be superfuel though...
But so much doesn't make sense because from Chakotay & Tuvok's recap, we hear:
10 months 11 days ago- Voyager landed on Class Y planet in Vaskan Sector. ("Demon")
9 months 28 days ago- Voyager got silicate from a comet in the Podaris sector.
Then came the N'Kree conscription (
9m 2d), Kmada sabatage with low level theta radiation (
8m 17d).
(Note: Voyager never encountered the N'Kree or Kmada, or anything Podaris [Vaskan named after the race in the "Living Witness" episode], but we hear no mention of Borg combat from Mimic Voyager or Devore or the Void, so they probably sped over the areas Voyager went slow in)
The nebula was 110ly across. That would add so much time going around, Janeway decided to go through it in stasis to avoid subnucleonic radiation at a warp speed I forget, but it took exactly 1 month. I would presume the Silverbloods weren't affected by it, so cruised thru it. They would have to have gone through at higher speeds.
Other factoids for any math in case any people more mathematically adept want to number crunch to deduce where each event in this episode was and how far they were ahead of Voyager... or to make a handy map (Star Trek Star Charts is a handy starting point, but it has a myriad of small errors):
- The episode
starts at 52586.3. They say at peak efficiency & no stopping, they
will reach Earth in 2 years 11 days 6 hours. Of course, Janeway is curious about a few things. And they did say they can pass straight through the center of the galaxy instead of around (but with the same point of origin, well before the center, and the same destination on the Alpha-Beta border, how far off the same course would both Voyagers be?)
- Mimic Voyager's
long range sensors can detect major phenomena 6 months ahead and 2-3 months round trip off to the side (anomalous gradient in the curvature of space and a binary system with unusual bioharmonic readings). Another system upgrade? Or is that anomalous curvature a gigantic feature Voyager skipped over in their 20,000ly leap in "Dark Frontier", so never got remotely close to detecting even from a distance?
- Tuvok said
anything new from the previous 30-40 weeks shows no decay. That's when they look back into the logs beyond then (above). We can use that 10 months 11 days for any time/speed/distance calculations.
- When Chakotay tells Janeway to go back to the Demon Planet, she says that's "
thousands of light years in the wrong direction".
- Neelix picked up
Hazari ale along the way. The Hazari were the lizard mercenaries in "Think Tank". We don't know how wide a region of space they operate across.
- That
Ord'mirit Class Y planet was the nearest Class Y planet detected on their sensors. It was reached by regular warp after they stopped using it to investigate the decay. After the battle, she activates the EWD and sets course for the Demon planet.
- After that, in an
undated supplemental, we hear they are
5 weeks away from the Demon planet.
- An unknown time later, Kim says it's
52597.4. They detect Voyager 22ly away at this point. (We don't know *when* they got enhanced warp drive)
- Voyager said they picked up the distress call at 0900 and arrived at the ship at 2120, though it's a flaw if it's the original distress call from the Ord'Mirit Class Y planet because that was a long time ago and far away (unless the distress call was continuous, not just the one use).
- Before or after "Dark Frontier" and its huge leap?
"Dark Frontier" aired 2/17/99, "The Disease" 2/24/99, "Course: Oblivion" 3/3/99. Disease was produced before Dark Frontier, but C:O was produced after. So, it should be after that huge jump.
Any math nerds can produce something cool out of this and the various distance leaps over "Hope and Fear" through "Dark Frontier"?