Though her lead reached nearly a minute at its peak, she would not be able to hold on. The other elite runners advanced on her, and at the 30-kilometer mark, they overtook her.

She didn’t make it much past 20 miles, dropping out after having the lead at every mile through No. 18.

“Just sharp pain in my calf,’’ said Smith, who is from New Zealand, attended Providence College, and now lives in Providence. “I nearly fell over a couple of times.

“I tried to change the way I was running for a few miles, to see if it would ease. It would go on and off, and in the end I just couldn’t run any further because it was really painful.’’

It was easy to spot Smith through the first half of the race, ahead of everyone else, wearing bright blue shorts. She went out at a pace that the top runners decided not to challenge.

“I think that pace was a little too difficult,’’ said second-place finisher Desiree Davila.

But that was Smith’s plan, and she was pleased with the way she executed it, until the discomfort started.

“I’ve just been feeling good and training well, and I didn’t want it to be a slow race,’’ Smith said. “So I decided to just take it out. That was the plan that we had.’’

Said American Kara Goucher, who finished fifth, “The only time it seemed like we might panic and try to catch her was early on, at 5 [kilometers], at the first water bottle stop. And then things settled right back down.

“I just felt like she went for it and that’s great. But I was looking around, and Desi was still there, there were past champions that were still there. I felt like she was going to get reeled in.’’

But no one expected it to happen the way it did.

Over the next 5 miles, she tried to keep up her pace, struggling into the Newton hills. She bent down a couple of times, her hand going to her calf, even stopping for a few seconds. By the end, she said, it felt like something in her calf was tearing at every step.

“I couldn’t run anymore,’’ said Smith, 29, who was running her fourth marathon. “I couldn’t physically run any further.’’

It was pain unlike any she had experienced, pain that mystified her after the race. She was examined by doctors but still had no idea what happened. She consumed liquids and gel throughout the race, so she didn’t think that was the issue.

“It’s really disappointing,’’ Smith said. “I felt really good. And then it just came on so quick.’’

Across the top, the flag read, “Keep trying, Japan. Keep the courage.’’

Both winners said they were inspired to excel after the earthquakes and tsunamis devastated their homeland.

“With everything that’s happened in Japan, I really wanted to try hard this year, especially to finish in Boston and especially to do well for my country,’’ said Soejima, through interpreter Kay Horiuchi.

Soejima, who wore a sticker on his jersey that read “strength and courage,’’ outpaced Ernst Van Dyk, who was attempting to win Boston for the 10th time, to win in 1:18:50. It was his second Boston victory.

At the end of the race, Soejima said, he was thinking, “Until my hands start bleeding, until my heart stops, I’m going to try until the very end.’’

Tsuchida, who lives in Tokyo, had her training disrupted by the disasters and had to travel to three locations to complete her preparation for the marathon. She finished in 1:34:06, a course record.

Tsuchida said at one point she thought she might not make it to Boston.

“Initially I was very scared that I wasn’t going to be able to come, but I overcame that and I wanted to come to Boston,’’ she said. “During the hard times [in the race], when I wanted to give up or I was feeling a little down, that’s when I had the courage from the folks in Japan.

“I don’t care if I cut my fingers or anything, I knew I wanted to finish.’’

Michael Whitmer and Shira Springer of the Globe staff contributed to this report.