Or a Short One

[dukealien]

Ah, well, glad the images got across.  As for the rest, visualize this:  big slug wanders around in the rough for years, so this Long March becomes part of its vocabulary.  But after a while, it conceives the idea to make a change of course and get to the rich green lawn on the other side of the sidewalk.  So up it climbs, and soon the promise is in sight... but instead of continuing across, it swerves as only a slug can swerve (left or right, doesn't matter) and proceeds down the middle of the sidewalk, progressively losing substance but unwilling to complete the change it began with such high hope.

Something like that.