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I am confused by Season 5 Episode 1!? Help Me!

So heres a question to test your knowledge but honestly speaking i missed a scene or an episode so i need to rewatch it to make things clear. I will explain

I am currently in Season 5 episode 1. Chakotay was discussing with 7 of 9 (Borg Lady) that they have been travelling for months through the expanse and that he doesnt know how they will last 2 years travelling.

I thought they were going to take like 50 years or so to get back to Earth. It seems i missed something out and im unable to connect the dots. Did they get a new engine or something? Did the again get thrown ahead in space (but that far?)

The question is which episode made them reduce their time to make it to Earth? Please connect the dots for me!! Thanks!