Well, time to sharpen those guitar playing skills. Found this on Youtube looking for the guitar tab for the main theme used for Byousoku 5 Centimeter:

Pretty unusual to put a guitar tab in this format, but it is quite useful at making you play at the original song's speed. Unfortunately, at my skill level, it's nothing to do but repetition combined with getting some finger blisters as I don't play as often as I should1.

1. 1. And I should play guitar more often. It's free to do when I already have the equipment unlike shooting guns or playing hockey and, even if I'm not skilled, it's just as therapeutic.

For those looking for it, the guitar tab is available on this topic in the forums on Ally Kerr's official website in PDF format. Though this is for a single guitar; what you hear during the intro of Mushishi is actually two guitars.