The Marathon is a huge fundraising event for British charities and many people run on behalf of charities, donning a variety of costumes to do so. I've seen The Angel of the North (how anyone could run with such a sculpture on them is beyond me), a UK Mail parcel, and a rhino. You have to applaud the sheer bravery of someone willing to draw a rhino costume along London streets!
I'd love to run the Marathon. I even applied a few years ago, but it's super hard to get in. One day!