Meredith Fitzmaurice says the farthest she's run is about 32 kilometres. She completed 42 km in the winning the race. (Francois Lenoir/Reuters)

Meredith Fitzmaurice had planned to run the half-marathon at the Run for Heroes in Amherstburg, Ont., but missed the turn and became the first woman across the finish line, qualifying her for the prestigious Boston Marathon.

"I was training to do Detroit," Fitzmaurice told CBC Radio's As It Happens.  "It was a beautiful morning and it was perfect running conditions and I felt good. Twenty miles (32 kilometres) is the farthest I've ever done."

Fitzmaurice, 34, said she had figured her time for a half-marathon (21 kilometres or 13 miles) would be about 1:29.  When she was still running at 1:31, and not seeing a finish line for the half, she just kept going.

"I saw two men farther up ahead of me so ... I followed them," said Fitzmaurice.  "I had no one to talk to."

She said that by the time she reached the final stretch, people were yelling at her: "You're in first!" That's when she realized she was about to win the race.

Her time was 3:11:48 — enough to qualify for the Boston Marathon.

"It was a very happy day for me."