Good packing is a problem-solving exercise. Every item should answer one of a few questions: will it keep you safe, keep you moving, help you navigate, help you eat, or solve a likely mechanical problem?
Pack by function
Separate navigation, clothing, food, repair and emergency items into small groups. The goal is to find what you need in the dark without unpacking everything.
Plan for the worst realistic weather
Weather can change faster than pace. A compact waterproof layer, insulation appropriate to the route and dry backup items can be more valuable than another performance gadget.
Use the kit before race day
If a buckle, pocket, light mount or bottle system is annoying in training, it will be much more annoying after twelve hours.