You have a point Mark, one that has been raised by forumites in the past.
One view is that the r*nner and the charity both gain, so everyone is happy. The other view is that it is unfair to ask friends and family to pay so that you can do something you wanted to do anyway. Each to their own.
It's also a sad fact that some causes are more emotive than others. For instance, like it or not, sick kids will tend to tug at the heartstrings more than care of the elderly. On the whole I think this is reflected in the amount asked for by the charity.
Good luck to you all.