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Ginger cake. I make it for all my clubs running events and it always goes down well. It even got a mention in a RW review once!
Fruit cakes are popular too, but you need to cater for those who don't like fruit.
Flapjack - good for refuelling and for eating standing up!
I think any cake that is fairly solid without icing is a good bet. Icing makes things messy and sponge cakes tend to fall apart if you haven't got a plate to eat from
CUp cakes - easier to hand out
Made carrot &orange with cream cheese frosting this weekend. Not easy to hand out but it still dissappeared fast at work
Flapjacks also don't require any real cooking.
At the end of the race? Chocolate. Plenty of it!
Banana loaf or carrot cake or perhaps muffins - they're easy to make and hand out as well. Banana and choc chip go down well
Flapjacks as the others have said are also easy to make and eat!