Gloria, thank you. Tomorrow marks a big day for Boston. Their marathon moment one year after the run was shattered by the of riffic terror attack. Tonight, ABC's Dan Harris back in his hometown where... See More
Gloria, thank you. Tomorrow marks a big day for Boston. Their marathon moment one year after the run was shattered by the of riffic terror attack. Tonight, ABC's Dan Harris back in his hometown where runners are ready and security never so high. Reporter: From the finish line, where they're allowing the public tonight. Tomorrow's event is huge. They're expecting 9,000 more runners this time than last time. And upwards of 1 million spectators. Double the average. Obviously, one top priority is making sure we don't have a repeat of last year's attacks. Today the annual blessing of the runners. And meanwhile, underground, law enforcement officials are monitoring feeds from 100 cameras set up along the 26.2-mile marathon route. Double the amount of last year. Anything of trouble, any person that is presenting a concern to us, may be monitored very quickly. Reporter: Above ground, an unprecedented security operation. There will be 4,000 cops on the streets tomorrow, 500 of them undercover. For people directly impacted by the bombing, it will be a day full of emotion. Heather Abbott who lost the lower part of her left leg when the second bomb wept off, will run the last half mile alongside a woman who found her on the ground and helped save her life. Do you have a sense of what that will feel like for you? I can only imagine that's going to feel invigorating and sort of like a sense of closure. Reporter: David, I think it's safe to say, there are two top priorities when it comes to the 118th returning of the Boston marathon. First, safety. And second, making sure that the world knows that even though Boston got knocked down last year, it's back up and stronger than ever.
This transcript has been automatically generated and may not be 100% accurate.