At some point between drawing a ballot place and crossing a finish line, the London Marathon stops being an ambition and becomes a fact of someone’s life. The alarm at 5:30 on cold January mornings. The long Sunday run that started in the dark and ended in the light. The blisters, the foam roller, the precise calculation of when to eat and how much to drink. Twenty-six point two miles is not a distance. It is a project — one that takes over the training diary, the social calendar, and a significant portion of the mental bandwidth of the person…