Tom and Carole Fabian hope to be among the London Marathon finishers. The couple, who grew up in central Massachusetts and now split their time between Lynnfield and Port Charlotte, Fla., ran in last Monday’s Boston Marathon. Tom finished in 4 hours 1 minute 34 seconds, then walked to the family reunion area near the John Hancock building. That was where he stood when he heard and felt the two bombs explode. Carole was stopped at Cleveland Circle. As they approach the finish on the Mall on Sunday, they will be thinking about Boston and keeping their eyes open.

“I don’t imagine anything is going to happen here,” said Tom. “But I’ll watch the side of the roads. The more people that are alert, the less likely things can happen. I ran down Boylston Street and was forced down that left side of the road. Was I looking for a bag? No. I was looking at the finish line. This time, when I finish I’ll be looking not so much at the finish line, but at the ground and what’s on the side of the road.”

In an ever-vigilant city all too familiar with terrorist bombings, Tom Fabian will be far from alone. The London Marathon is accustomed to heightened security. And the capital has a long history of forging ahead after acts of terrorism. Hosting 36,500 runners and an estimated 700,000 spectators less than a week after the Boston Marathon bombings, London will rely on lessons learned from its tragic past.

Started in 1981, the London Marathon grew up under the threat of terrorism with Irish Republican Army bombings regular occurrences in the 1980s and ’90s. On July 7, 2005, bombers attacked the London transportation system with three explosions on London Underground trains and another on a double-decker bus. Former race director Dave Bedford said the marathon has dealt with direct threats in the past and one nearly delayed the start. Also, he recalled that one year, a week before the race, city authorities quickly repaired part of the course after a bomb went off nearby.

“London was under attack from the IRA for probably the first 15 years of the race’s existence, so we’ve always worked very closely with the Metropolitan Police and the city politicians to ensure that the course is as safe and secure as possible,” said Bedford, who served as race director from 1998-2012 and currently recruits the elite field. “With 26 miles, it’s not the easiest thing to do, but we’ve always felt we’ve been on top of security.”

In advance of the marathon, the Metropolitan Police announced that it would be “deploying an increased and highly visible police presence” that would include “several hundred additional officers on the streets” and more search dogs. The police said that the additional officers were meant to “reassure the public attending Sunday’s London Marathon” and that there was no change in the threat level to London and “nothing at this stage to link the Boston bombings to the London Marathon.” Officials also cautioned spectators not to leave their belongings unattended.

Today, London is widely considered the most heavily surveilled city in the world with a total of roughly 500,000 closed-circuit television cameras and security video cameras. Perched on building corners, atop traffic signals, and throughout London Underground stations, the cameras provide blanket coverage of city streets and other major transportation routes. In the marathon command center, race organizers can use thousands of CCTV cameras along the course to monitor the event, according to London Marathon chief executive Nick Bitel.

“It’s against that background that we’re able to say we have confidence in our security,” said Bitel. “But we also had a review to see what else we needed to do after Boston. You’ve got to have a presence, but we don’t want people to be afraid. There’s always a balance.”