Many were past participants in the most famous marathon the world and were saddened by the scenes of carnage and death at the finish line.
"It's upsetting. It feels like somebody's attacking your friends and your family. My immediate thought was concern for my friends and my family who were down there," said Alison Zimmerman.
Doug Smith, of the Ontario Masters Athletics organization, said it's not just the runners that make a marathon.
"What makes marathoning special? It's the spectators - and those are the people killed and injured. And so just doing our little bit here sends a signal, I hope, to them that we're thinking of them," he said.
The run started at Queen's Park, loosely organized by runners using social media.
Runner Alyson Schmidt spoke for many when she said she thinks the bombings will only make them more determined in future.
"Runners are really strong and I think everyone will bind together and make sure that our Toronto [Goodlife Marathon on May 5] race is successful, and safe, and the spirit of running isn't affected."