Race number and safety pins - essentials! I always take a few extra safety pins as someone else is bound to need one and they're so easy to lose.
Timing tag and ties - essential if you want a time and for others to be able to track you around the course.
Pre-race snacks - Nakd bars are my favourite pre-race snack and I'll take a banana and some water too. Most organised races provide water en-route so there's no need to bring loads of water with you.
Tissues and hand sanitiser - because porta-loos are disgusting even when people are stone cold sober.
Blister stick and vaseline - feet saviours for last minute top-ups!
Phone, Oyster card and cash - for last minute texts / Instagrams and for post-race meet-ups. Some cash just in case - some races will have water and snacks for sale at the start but leave the rest of your wallet at home.
Old clothes for the start - the start might be cold so pack some layers which you don't mind discarding at the start.
Cap and sunglasses - these probably won't be coming with me tomorrow but I dug them out just in case it was going to be a sunny day!
iPod - my iPod shuffle is loaded with my favourite running songs.
Suncream stick - I'm an optimist and a huge huge SPF devotee. Although depending on how grim it is at the start this may be checked in.
Hairbands and lipbalm - these hairbands double as bracelets so perfect for mid race hair adjustments and SPF lip balm will be popped in my race belt too.
Energy Gels - always always the ones you have trained with to avoid and race day disasters...
Chewing gum - I like chewing gum while running - it's a nice change from the sickly energy gels.
Face wipes - because sweaty face is not a good look.
Change of clothes - pack a change of clothes of give a bag to someone you're meeting at the end - there are too many post-marathon free food deals to waste time going home to shower!
Snacks - a lot of big races give you snacks in the goodie bag but if you know you'll want a specific snack then pack it!
Flip-flops - again, weather dependent but for hot races I always swap trainers for flip flops straight after.