Most decent size races will have a drop bag where you can leave your stuff. I tend to have a spi-belt for my mobile phone, and I put my bank-card, my car-key and my ID (driving license) in the little pocket that is usually in my shorts/capris.
All I carry is my car key kept in a zip pocket of my shorts or running tights. The only addition is a gore waterproof/wind coat which if I need I remove and carry in a little bum bag..organised or what
In general I carry a car/bike key in a flipbelt and phone (if leaving my stuff on the bike in a pannier rack) - for longer fell races where full kit is required I'll be carrying a small rucksack/running vest anyway.