It's impossible to physically secure every foot of the race route, but officials hope citizens will keep their eyes open as well.
A lot has been said about the increased security at this year's Chicago Marathon, but can police really secure a route that stretches more than 26 miles through dozens of neighborhoods?
It's impossible to physically secure every foot of the race route, but a group called the Front Runners will be joining 230 volunteers in staffing the aid station in Lakeview on race day.
"We're going to do a gear check within our own water station to help prevent backpacks on the ground, stuff like that," volunteer David Reithoffer said. "It think we're just going to be a little more aware of who's around us and what they're doing."
It's that type of citizen vigilance police officials hope will help fill in the gaps.
"If you're asking somebody to say is there going to be 100 percent absolute security, I don't thing the answer to that is ever yes, but you're going to see it at the highest level probably in the history of marathons," Chicago security expert Tom Kasza said.
Kasza helped protect four presidents as a Secret Service agent, but on Sunday he'll be a spectator in his West Loop neighborhood and expects many more to watch the event like he was trained to do.
"They're certainly no longer going to tolerate seeing something and ignoring it and putting everybody in jeopardy," Kasza said.
Beyond the new heavily-secured start and finish lines, police have also pinpointed potential risk areas along the entire route and created plans to minimize them.
Even veteran runners say they'll approach this marathon differently.
"You have to have it in the back of your mind, you know, because it happened," runner Corbin Woodling said.