BOSTON — Joan Benoit Samuelson looked around the town common in Hopkinton on Monday morning before the start of the 115th Boston Marathon and instead of that familiar steely determination, there were question marks in her eyes.

She wanted to know the location of the buses headed back to Boston, in case she decided not to run.

“I’ve never,” she said, “been that indecisive before a big event.”

Eighteen years had passed since the last time she ran from Hopkinton to Boston on Patriots Day, but only four days had elapsed since she awoke Thursday morning with back spasms.

The day before, she had helped put on a clinic at the Reggie Lewis Center in Boston for inner-city youth and led four 20-minute stretching sessions. Stretching has never been a big part of Samuelson’s routine, and her 53-year-old muscles, hardened from a long Maine winter, are more gnarled oak than whispering willow.

“I stretched too much,” she said. “Then I drove back (to Freeport) in that driving rain, white-knuckled, and the next morning I woke up and was in total spasm.”

She sought treatment. Got conflicting advice. She had never dropped out of a race before and certainly wasn’t about to do so at Boston, where she had won in 1979 and 1983.

This was also to be daughter Abby Samuelson’s first Boston Marathon, and they rode a bus together to the start. One of the elite women, Sheri Piers of Falmouth, saw the doubt in Hopkinton.

“I felt bad for her because you could tell she didn’t want to back out,” Piers said. “But she’s like, ‘I don’t want to start, either.’ “

The sun was out. Wind from the west was pushing runners toward Boston. Samuelson warmed up and decided to give it a shot, starting with the first wave of 9,000 runners at 10 a.m., about half an hour behind Piers and the rest of the elite women.

“I didn’t really feel at ease until the first 10K,” Samuelson said. “Then I started to get in the groove and, somehow, the miles disappeared between 10K and about 20 miles.”

Because she ran within two minutes of the Olympic Trials qualifying time in the heat of Chicago in October, Samuelson knew many in the media suspected a sub-2:46 time was in the back of her mind. She knew differently, having spent the bulk of her training time on snow instead of roads, and having taken an arduous trip through the French and Swiss Alps at the end of March.

“Then, with this back,” she said, “it was all about finishing.”

Finish she did, in 2 hours, 51 minutes, 29 seconds. Forty five women and 611 men crossed the line ahead of her; none of them has yet to celebrate a 53rd birthday, as she has.

Samuelson gave a brief television interview after the race, then ducked into the medical tent. Those last six miles had not been easy. More survival than celebration.

The back “did shut me down earlier than I would have liked,” she said. “That was sad. I really wanted to stride out (past the thick crowds lining downtown streets) but I just couldn’t. I was finished by the time I got to Kenmore Square.”

After rest and recovery, she went out to welcome Abby, now 23 and living in the other Portland, the one in Oregon. Mother met exhausted daughter in Copley Square and planted a kiss on each cheek.

Abby said she made the classic mistake of going out too fast and paid for it after the hills of Newton. She finished in 3:30:36 after, for the first time she could remember, having walked part of the course.

Her next marathon?

“I’m going to wait awhile,” she said. “That’s for sure.”

As for her mom, this won’t be her last footrace from Hopkinton to the Boston Public Library.

“It was great to be back on the Boston course,” Joan said. “It’s been 18 years. I’ll do it again when I can run with reckless abandon.”