Several of my friends and acquaintances have run the Boston Marathon in previous years. For those who are, "in to," marathons it is one of those venues that aficionados like to tick off their list. Thankfully no one that I knew decided to take the trip across the pond this year.
At first glance it seems strange that a running event should be the target for the actions of some terrorist grouping or malcontent. But if you want to cause panic, if you want to cause fear, if you want to have the attention of the media, then selecting a large sporting event which takes place on public roads is probably going to bring the results you desire. Providing security at such a venue is always going to be difficult and involve compromises. This will be very much in the minds of the organisers of the London Marathon as they prepare for Sunday's race.
It is extremely unlikely that the perpetrator or perpetrators of the carnage in Boston have London in their sights. Where the greatest danger may be is in the actions of some individual who decides upon a copycat attack