Don't forget that he's also done the Comrades ultra in both directions. He knows about training, finding the time is entirely another challenge. I have huge respect for the man. I met him at the London marathon start one year, when he gave a right dressing down to Jade Goody (Remember her? No? No surprises then).
When he does London marathon he is doing is as a public race against Michel Roux junior. I find that encapsulates the spirit of the mass event.