Joshua Cassidy knew he'd win his first Boston Marathon once he reached the top of the course's Heartbreak Hill with the lead in the men's wheelchair race. He was less certain about a world-record time, though it was in reach. 

"I just started hammering the rest of the way," Cassidy said. 

It paid off when Cassidy, who was born in Ottawa but lives in Toronto, crossed the finish line in 1:18:25, which beats the world record by two seconds. 

Ernst Van Dyk of South Africa set the previous record in Boston in 2004. 

Cassidy, 27, led from the third mile and finished comfortably ahead of last year's runner-up Kurt Fearnley of Australia, who finished second in 1:21:39. 

Cassidy said when he reached the bottom of Heartbreak Hill at around the 18-mile mark, he figured the race was his if he was still ahead of Van Dyk when the climb was over.

"(Van Dyk) is the best climber in the world by far," Cassidy said. "I knew if I could just get to the top first, I'd have it for sure." 

Cassidy then switched his cyclometer from speed to time and went for the record, knowing only that it was somewhere in the 1:18:00 range. 

Cassidy notched the record despite temperatures around 30 C at the time of the finish, and while wearing the long sleeved compression shirt he trained in. 

"I wanted to stick with what was familiar and not worry about the heat," he said. 

Cassidy won the London Marathon in 2010 and said Monday he'll compete in that race next week. 

The women's wheelchair race was far tighter than the blowout men's race. 

American Shirley Reilly edged Japan's Wakako Tsuchida during a sprint to the finish line. 

Reilly finished in 1:37:36 on Monday, one second ahead of defending champion Tsuchida. 

Last year, Tsuchida beat Reilly by nearly seven minutes. 

Reilly and Tsuchida battled throughout, with Tsuchida briefly breaking away around the 18-mile mark before Reilly closed the gap.

Diane Roy, 41, from Lac-des-Aigles, Que., finished third, five minutes behind Tsuchida.