Some of these items appear here after I learnt the hard way…
Race number and safety pins - or use a triathlon belt. Don't forget to fill in the medical / contact information in permanent pen.
Timing chip attached securely to trainers.
Race documents (maps, directions).
Bag for your kit.
Keys, wallet, charged mobile phone.
Comfortable training shoes.
Socks you have already run long in!
- It's a great idea to run long in the clothes you plan to wear, some tops or shorts that are comfy for 10k can be hell at 30k.
Also pack just in case
High factor waterproof sunscreen.
Body Glide/Vaseline. Think about using these around your toes, inner thighs, and where arms brush torso. Especially it its hot these areas will form salt crystals that can rub and can eventually really hurt. Gentlemen's parts might also appreciate a bit of help with anti friction.
Plasters - If you get joggers nipple, though selection and testing of the right clothing might
Toilet paper in plastic bag.
Sports watch or heart rate monitor.
Charged iPod or MP3 player.
Medication (ex: asthma inhaler).
Fuel (gel packets, race snack/recovery drink for after race).
Comfy shoes (I pack my old Birkenstocks).
Clean dry socks.
Recovery drink (Protein based).
In case of warm weather
Consider wearing a runners visor or headband.
In case of cold weather
Throw-away tshirt (or that jumper Aunt Mavis gave you at Christmas) for first few miles.
In case of rain
Light Waterproof Jacket.
Trashbag to wear pre-race.
Have I missed any things that you would not be without?