Honestly, the whole Warp Factor speed conversion didn't even attempt to get nailed down until the non-canon Star Trek Tech Manual was printed; and even hat was an arbitrary idea that later Star Trek series took and ran with.
Warp drive allowed them to travel at the 'speed of plot' for quite a while. Hell, in
Where No Man Has Gone Before it's infered that Impuse Engines can drive a ship at FTL speeds as when Sock makes the comment that the Valiant was 'caught and swept into/thrown clear of the barrier, Kirk comments "The old Impulse Engines weren't strong enough"; plus the fact that Spock indicated that they were thrown 1/2 a light year out of the Galaxy and 're-entered here'. Now, if the ship only had Impuse drives, that would have been 6 months before they were able to re-enter the galaxy; but if they had a crewmember like Gary Lockwood's character, we can assume the build up to the captain destroying his own ship took a week or so, like it did on the 1701.
Then we have episodes like
That Which Survives where the 1701 is thrown 994.7 light years; yet makes it back to the planet it was thrown from (and yes they were travelling at Warp 14 for 15 minutes or so) in about 48 hours. Maybe Scotty REALLY WAS a miricle worker.
Bottom line, there's no way to reconcile anything like this without some 'out of series' rationalization. Not that this is a bad thing, and Star Trek fans used to love coming up with explainations for waht were refered to as "YATI's" or
Yet
Another
Trek
Inconsistency
