The big cities are actually quite english friendly and at any train station or hotel you could find an attendant who speaks english. Signs and announcements as well usually alternate between a japanese and english version.
To actually answer how much you should know, I would recommend focusing on vocabulary and kanji and stick to basic grammar. I found it was more important to know what words people were saying rather than the relation between them. And kanji is very necessary because most signs are written in kanji, and unless you are reading something written for kids it will not have the furigana above it. Also many names, especially that of places, are kanji and many are just expected to be known by even little kids just because they are ubiquitous (words like come and go).