LONDON (AP) — Only a few days after seeing up close the carnage of the Boston Marathon bombings, Mark Cliggett is gearing up — mentally and physically — for another long run Sunday in London.

Cliggett, a 51-year-old software engineer from Seattle, and his wife were about 200 yards from finishing Monday's race when the bombs exploded, killing three people and injuring more than 180.

Witnessing all that has had an impact on Cliggett, and millions of other runners around of the world. But none of it will stop him from running his second marathon in a week through the streets of the British capital.

"Running is running," said Cliggett, a marathon veteran who raised money for cancer research in Boston and will be running to raise money for kids in need of wheelchairs in London. "Yes, it has affected me overall. It's dominated my thoughts since then, and from reading runners' forums it sounds like that's happening to everyone who was there."

"But mostly," he added, "(I'm feeling) just incredible sadness for the people affected."

Three people were killed at the scene, including an 8-year-old boy. On Friday, police in Boston killed one of the two suspects.

John DeMuria is another American who will run in London. Unlike Cliggett, the 55-year-old businessman from just outside New York will be running a 26.2-mile course for the first time. He, too, won't be cowed by what happened in Boston.

"Quite the contrary. It's a sign of solidarity, particularly as an American running in London," said DeMuria, who is running to raise money for children's cancer. "I'm not afraid."

DeMuria and Cliggett will be wearing black armbands during the race to honor the victims of the Boston attack. And 40 percent more officers than last year are expected to be on duty Sunday to ensure that a similar attack doesn't happen in London.

But both know that protecting a course that winds through one of the biggest cities in the world is impossible.

"I'm not stamping my foot and saying, 'Do something.' I do believe people will make adjustments. I appreciate that," Cliggett said in an email to The Associated Press. "Especially for the spectators, who are more at risk than the runners. Runners have to be very unlucky to be affected personally by something. Spectators are unfortunately more predictable in terms of security risk."

David Corfield, a 32-year-old Londoner who works in advertising, will also be running a marathon for the first time. Like the others, he sees no reason to pull out because of Boston.

"I think it's probably a marathon runner's makeup, a bit of defiance," said Corfield, who is raising money for cancer research. "I don't think you can stop for this kind of thing. You've got to carry on. You can't live your life in fear."

Besides the professional athletes who will be competing for money and glory, more than 35,000 runners and a half-million spectators — including Prince Harry — are expected to be at the London Marathon.

The race will start in east London at Greenwich Park. The course later crosses the River Thames on Tower Bridge and takes in other London landmarks, including Big Ben and Parliament. The end is near Buckingham Palace.

The finish line, of course, is the goal for everyone. But Cliggett, who has run dozens of marathons and several ultra marathons, didn't get to actually cross the line Monday.

He and his wife, Janet Vogelzang, were on the last stretch of the race, coming down Boylston Street. They were a little behind schedule, slowed by a problem with Janet's contact lens.

Then came the blasts.

"Very soon after, the police came over, told us the race was over, and told us to go back the other way," Cliggett said. "Someone asked if they could finish, which surprised me, although I guess it could have been someone's first marathon or first Boston and they were still locked on that vision of the finish line.

"Compared to what the victims and their families-friends are going through, not finishing officially doesn't even register on the scale of things to think about."

Cliggett said he saw one man with shredded pants and bloody legs being helped into a vehicle. But there wasn't much he could do after initially working to protect himself and his wife, and then being told to head the other way.

"This is the thing I've been thinking about," Cliggett said. "There was a very small time between being completely terrified and being ushered away by the police. But in those moments could I have helped? I will say that I'm completely in awe of the people who rushed in immediately to help. The word hero doesn't do those people justice."

For the race in London, the biggest hope is that "heroes" won't be needed at all.