I would agree on the charity walks too – it’s got to be some minimum level of effort, like my sister walking from Exeter to Winchester with a friend. If you would conceivably do it for fun, I don’t see why I should sponsor you to do it!
That said, a group of cyclists at work did a sponsored ride from Bristol to London for the Pakistan flood appeal this year. At least one of them had been previously heard to say something along the lines of ‘they bloody deserve it’. He just wanted an excuse for a day off work on his bike.