"Let's go farther."

Translation: Chodźmy dalej.

[LidkaAndr]

is there any difference between further and farther?

[tadjanow]

http://www.quickanddirtytips.com/education/grammar/further-versus-farther

In short, 'farther' is only used for physical distance, while 'further' is used in figurative sense.

[LidkaAndr]

Thank you :)

[MarjanvanK]

what about : idźmy dalej. ?

[Jellei]

I think it technically is okay, but definitely not as common as "chodźmy dalej". It works.