In reply to Enty:
The answer is ... it depends.
For a lot of runs up to about 90 minutes, I probably wouldn't carry anything, although if it's 90 minutes in the hills, I'd maybe take a bumbag with a windproof jacket, a couple of gels and maybe some water. A lot would depend on the weather. Flexible water bottles are quite useful for this, since they can be made to fit the available space better than rigid bottles.
I've also got a couple of running tops with pockets on the back, like cycling tops, which are quite good for carrying a few gels.
For longer runs, I've got an Innov-8 rucksack with an inbuilt bladder and space for waterproofs, food, drink, map and compass.