Many an Ironman race has been decided upon over a few too many pints with friends - and often, with little to no experience these people go through with it (just!). This could just as easily be the same case if you took away the celebrity status.
At least he'll have a support crew to get him to the start line (unlike jo blogs) and if he has the time to train (a lot!) over the next few months he'll get to finish it too. I don't care that he's a celebrity myself. Though he is raising the profile of our sport and hopefully it will help bring more people in wanting to have a go.
Also, it's for a charity don't forget so lets hope it makes lots of money for the cause. Another reason to applaud him for having a go. Good luck to all involved.