I've always run with some form of bag with kit in it, mainly because I run alone away from people. So if I end up injured, out in the middle of nowhere, at night, in fowl weather I should be able to survive. It may sound a bit dramatic but it's quite easy to get hypothermia, even in the middle of summer.
Of course I don't go for a 3 miler carrying a 50lb pack, but at the very least I'll take a phone.
With an 85 miler planned for August in the south of England I've started running with something representative of the kit I might carry in the 24 hour race. Yes it's 7 months away, but it's important to get used to running with weight.
First off, the race is supported, with water and food stops every 10 miles (ish) and so my feeling is that I only need to carry enough food to get me to the next aid station, and I'll use the 1 and 1/2 litre bladder in the 12 Set. I'm not a great fan of gels, so I'll be carrying some real food instead. One thing I don't expect to get on the race is electrolytes so I'll need to carry them for the whole race, and have some way of taking them. I have some time to try some of the different products out there; I don't get on with Nuun, so I'm going to try good old Dioralyte.
Next on my list is protection. This is one area that can change right up to the start of the race depending on the weather forecast. But a waterproof jacket and lightweight sleeved top will always be in there; if I have to stop, changing out of my sweaty top and putting on the waterproof will keep me warm. There's scope for carrying waterproof trousers, a wind proof jacket, some spare socks, gloves and a beanie hat depending on how bad the weather will be.
Finally I'll need Body Glide, head torch, spare batteries, GPS, compass, route map, phone, sunglasses, sunblock, toilet paper, foot first aid kit.
There is another option. Rather than fill the 12 Set up I could sling a waterproof in my bum bag and carry a couple of hand bottles for hydration, but I'm not comfortable with that level of kit and my ability to complete the race.