In the TOS ep "By any Other Name" the Kelvan's were from the Andromeda Galaxy which is about 1,000,000 LY's away. If it would take Voyager about 70 years to get about 75,000 LY's. That means that it would have taken the Kelvan's without wormholes or other shortcuts about 900 years to get to the Milky Way Galaxy. Not to forget they would be travelling through interstellar space with nothing out there to resupply food, energy, and other vital supplies. This may have been the biggest streach in the History of Trek, IMO there would be no way (without one hell of a wormhole) that any species could cover that kind of distance without the crew and ship dying in deep space...