In daylight depending on visibility and how close you are to other runners you could get away with zero nav skills. However, if the clag's down and you're on your own some sections are by no means straight-forward, Fleet Moss and Middle Tongue in particular. At night you'll be grouped with three other people. As a group you'll need decent nav skills to avoid mistakes, especially as by that time you're likely to be completely knackered and in no mood for unnecessary detours!