I am not sure who your target market is. Its unlikely that the main appeal would be to existing "runners" but if it were family friendly you might attract a wider audience but 5km might be a bit far.
The link with the charity would be vital as this "Might" attract more people to turn up.
Have you thought about the actual logistics of this - you would need a huge area to organise it given the different paces people would be running at. Also a lot of equipment unless everyone did it in waves (ie how many space hoppers do u think you would need / get?)
I am sure for those who took part it would be a laugh but your main obstacle would probably be getting people to commit in the first place which is where the charity might come in?