Yeah, I know how you feel. I picked a song like this (really long (~3 minutes), not too many changes to the melody) and haven't bothered to finish. (or start, for that matter...)
I think the best way to do it is to push yourself just to get through the song once (with real arrows) and come back later to tweak it. Intuition is good enough, usually.
Also, you could try counting the number of times a musical phrase repeats itself, then work patterns for that. (For example, if it repeats an 8-measure section eight times, you know you have 64 measures for back-and-forth, mirrored, etc, patterns. Sometimes repetition is a good thing.)
Alternatively, find another song. You'll get the hang of charting once you finish a few.
In other words, it's okay to have a stack of WIPs.