In the wake of the deadly bombings that devastated Boston and terrified America on Monday, this may be a tough reality to swallow. But it's true nonetheless: Implementing airtight security on a 26.2-mile marathon course is nearly impossible.

Unlike stadium sporting events, which are by their very nature contained, access control poses an enormous security challenge at marathons. The Boston Marathon winds through 26.2 miles of suburbs as well as densely populated blocks downtown, and no gate with security personnel and metal detectors runs the length of the roadway. A half million people come and go as they please, lining the streets and filling large grandstands at the finish line.

This come-and-go atmosphere presents another problem unique to marathons — contact between athletes and spectators. Fans along the route hand out orange slices to runners and kids offer enthusiastic high-fives. Free-flowing access is part of the marathon culture.

"I've lost sleep over the fact that you have 52 miles of open roadway, 26 on each side," Guy Morse, who served as Boston Marathon race director from 1985-2000 and BAA executive director from 2000-2010, tells "That's the way I looked at it. You look at both sides of the road, as well as the course itself. It is impossible to secure it to the extent necessary. So, it has significant ramifications for major events."

Still, by all accounts, security in Boston on Monday was tight: Fire, emergency management, law enforcement, and race organizers met multiple times before the race to hammer out plans, form incident-response teams, and drill for emergencies. Police, safety officials, and bomb-sniffing dogs all inspected the roadway before the race.

And yet, the bomber (or bombers) succeeded.

London, which will host 37,000 marathoners this Sunday, is already on alert for copycats, reviewing security plans and keeping in close contact with runners, emailing them updates as the race approaches. This morning, Hugh Robertson, the British government's sports minister, sounded confident on BBC radio, saying he's "absolutely confident we can keep the London Marathon safe and secure." Let's hope so.