Massachusetts Governor Deval Patrick said security would be tighter than ever for the 118th edition of the world's oldest marathon, with the race for non-elite runners started at 3pm Irish time.
He told CBS: "There'll be considerably more police presence. But we also don't want to have it, you know, kind of a race through a militarised zone. So it's about striking a balance, and I think we have struck that balance."
Runners attending the event will have to use clear plastic bags for their belongings, and fans hoping to watch near the finish line are encouraged to leave strollers and backpacks behind.
More than 100 cameras have been installed along the route in Boston, and 50 or so "observation points" will be set up around the finish line "to monitor the crowd", the Boston Athletic Association said.
Mr Patrick said there have been no specific threats against the race or the city for the Massachusetts holiday of Patriots' Day.
"We're not taking that as a sign to sort of stand down," he said. "We're very prepared, and we're assuring people as much as we can that it'll be a fun day and a safe one."
About 36,000 runners have registered for the race - the second-largest field in its history, many of them coming to show support for the event and the city that was shocked by the attack on its signature sporting event.
Race organisers expanded the field from its recent cap of 27,000 to make room for more than 5,000 runners who were still on the course at the time of the explosions, for friends and relatives of the victims and for those who made the case that they were "profoundly impacted" by the attack.
Lelisa Desisa of Ethiopia and Kenya's Rita Jeptoo, who crossed the finish line on Boylston Street about three hours before the explosions, will return to defend their championships. Desisa returned to Boston last fall to donate his first-place medal to the city as a gesture of support.
Jeptoo, who also won the race in 2006, said she is hoping for a third victory - and one she can enjoy.
"It was very difficult to be happy. People were injured and children died," she said of last year's marathon. "If I'm going to win again, I hope I can be happier and to show people, like I was supposed to last year."