Meredith Fitzmaurice said she planned to only do a half-marathon at the Run for Heroes in Amherstburg, Ontario, but missed the turn, CBC news reported.
She didn't realize her mistake until she reached the final stretch and people starting yelling "you're in first!"
She was the first woman to cross the finish line, with a time of 3:11:48, good enough to qualify for the Boston Marathon.
Fitzmaurice said she had estimated she could do a half marathon, about 13 miles, in 1 hour 29 minutes.
When she didn't see a finish line at 1:31, she just kept running, following two men ahead of her.
Before that day, the farthest Fitzmaurice had run was about 20 miles.