Paula Radcliffe says spectators at the London Marathon will need to be more aware of security following the bombing of the Boston Marathon on Monday.
The three-time London winner was horrified by the events which left three dead and more than 150 injured.
Organisers say they will hold a 30-second silence and encourage runners to wear a black ribbon at Sunday's race.
"A lot will depend on people's vigilance and being aware what is going on around them," she told BBC Sport.
Radcliffe added: "I'm confident in the race organisers and the Met Police and they won't let it go ahead unless they are satisfied. But over 26 miles, you can't guarantee everything.
"It's a horrible situation to be in where you are looking at the people around you with suspicion."
A senior Metropolitan Police officer has reassured runners and spectators that Sunday's event will go ahead as planned.
Commander Christine Jones said security was being reviewed following two deadly explosions which happened near the finish line in Boston on Monday at the Boston Marathon.
Radcliffe also admitted she would have doubts about bringing her family to Sunday's race were she in the field.
She added: "I think first and foremost as a mother I'd think more about having family at the finish area.
"You put yourself there at your own risk but putting family in that situation is something people are going to have to come to terms with and conquer."
But she believes that the London race should still go ahead and competitors will want to take part.
"It is hard to comprehend that anyone could want to do something as cowardly as that and to target innocent people, especially children, in that way," she added.
"On the one hand, it seems trivial to be running a race after this but on the other hand, if you call the race off, are you letting terrorists win?
"Everyone knows the potential of marathons in terms of raising money for charity and how much would be lost if it can't go ahead and also how much good could be done to help those suffering in the aftermath of Boston."
Ribbons will be given to all runners when they pick up their race number.
Race director Hugh Brasher said: "We want to show our support for our friends and colleagues in Boston at this difficult time for the global running community.
"We are determined to deliver an amazing event that will focus on one of the core pillars of the London Marathon, which is 'to have fun and provide some happiness and a sense of achievement in a troubled world'."
Meanwhile, South African wheelchair racer Ernst van Dyk, a nine-time winner of the Boston race, has told of the scenes of horror he witnessed in the aftermath of the explosions.
Van Dyk, who finished second this year, and who will be also competing in London, was attending a sponsor's function at a hotel opposite the site of the second explosion on Boyleston Street.
"We were looking out the windows at the runners coming over the line and heard the first explosion up the street," he told BBC World Service Sport. "We couldn't see it and nobody was sure what it was and then the second explosion happened.
"It was a tremendous blast and the whole building shock. Immediately you could see people were hurt. I saw a guy and I'm pretty sure it looked like both his legs were severed - you couldn't see much of it.
"Panic broke out and people were screaming and crying. We were there for about five minutes before we were evacuated into another area and after about 90 minutes we were released and I went back to my hotel.
"The smell of gunpowder in the air was sickening. I've never experienced anything like it before.
"The people who were finishing at that time were the charity runners and their families were all there and there were a lot of kids who got hurt and injured. It is horrific and you can't imagine what sort of person would do that."