If you’re driving down North Road and happen to see a petite woman in a pink fleece, with long, straw-colored hair, be sure to wave. She’s always friendly enough to return the gesture, even nearing the end of a brisk 16-mile walk over hilly terrain.

The woman is Susan Larsen, and she’s training for a marathon of sorts. In May, she will participate in the Avon Walk for Breast Cancer, a two-day, nearly 40-mile walk through Boston.

A year ago, she’ll tell you, she never would have imagined herself in this position. “This is a person who hated athletics. I hated moving,” she said of herself this week, standing outside the West Tisbury School, where her daughter is a student. But all that’s changed. If it hadn’t been for the interview, she said, she would have sneaked off for a walk before the start of her daughter’s afternoon volleyball game.

The change began when Mrs. Larsen made a vow to herself to get in shape last year. But just as she reached her goal and felt she was in the prime of her health, the bad news came.

“I got physically fit,” she said, “and got the news I had cancer.”

The news was delivered during lunchtime at the Chilmark School, where Mrs. Larsen teaches grades four and five. She cried from the shock, at first. But she quickly recovered, and set out to get herself healthy again.

“My husband thought I was crazy,” she said, when she told him of her diagnosis. “He said, ‘you’re dramatic.’” But the breast cancer had been discovered through early detection at the Martha’s Vineyard Hospital, and the diagnosis proved accurate.

She was sent off-Island for further tests, including one genetic test to determine whether she would need a double mastectomy, likely to be followed by other invasive surgeries. Luckily, that test came back negative just days before she was scheduled for surgery.

Weighing all her options, Mrs. Larsen eventually decided to go through with a single mastectomy. And she got plenty of support from her husband and three daughters. “Abby said, ‘Hey, Mom, you know Amazon warriors way back when, they always cut off one breast so they could shoot their bows and arrows with precision,” she said of her middle daughter, a senior at the regional high school.

A week after the surgery, Mrs. Larsen found out that the cancer had progressed to the next stage, which meant a lumpectomy wouldn’t have been enough. “I made the right decision for me. Every woman is different,” she said.

Because she’d been in such good shape, Mrs. Larsen was able to return to work only three weeks after surgery. She did her best to get on with her life as it once had been, but it wasn’t long before she accepted that she was forever changed by the experience.

“It’s funny, when you get this and you survive, you think, ‘What can I do to give back?’ I had this nagging feeling,” she said. And soon enough, her question was answered. Mrs. Larsen got an invitation in the mail to participate in the Avon Walk to raise money for breast cancer. She liked the idea that the money raised would stay in the region, for breast cancer testing and research that could someday help her closest neighbors. So she went up to Boston, had a blast at the start party and signed up to walk.

“That’s all it took,” she said. “I’ve been walking since January 25.”

Mrs. Larsen tries to walk every day, favoring North Road and a spot in Aquinnah near Lobsterville Beach. She prefers to walk alone, as it’s hard to find company willing to walk the long distances required for her training. And there have, of course, been some ups and downs with the inclement winter weather. But she didn’t let it stop her.

“When we get dismissed early from snowstorms, I’ve got my husband’s [John Larsen’s] oilskins, because he’s a lobsterman. I walk in the snow, and I hide behind the trees when the snowplow comes,” she said.

During school vacation, she decided to engage in a little personal challenge, to help with her sponsorship. “I thought, let’s circumnavigate the Island, what the heck,” she said. “So I did, in three days. It was just kind of testing my endurance, and showing people that I mean business. When you donate to me, I’m going to work.”

She prefers main roads to trails, just in case she needs any roadside assistance. Plus she said, it’s lonely on the trails. “I like having people drive by, and saying, ‘There’s that crazy lady walking around.’ I prefer that actually. Isn’t that weird?” she asked, grinning.

The walks themselves have opened Mrs. Larsen’s eyes to a whole new side of her Vineyard home. “There’s so much you miss while you’re driving,” she said. “Little detail stuff, little gardens and ornate little fences. And the trash! I have to tell you, the trash is pretty cool.”

Trash talk aside, there is a part of her that feels like she owes this effort to the millions of other women afflicted by her disease, or other ailments. Three women in her town, that she knows of, are undergoing treatment for cancer. Two of her aunts had breast cancer. One survived, and one suffered an agonizing death. Mrs. Larsen’s roommate at Mass. General, a thirtysomething mother of young children, was diagnosed with terminal colon cancer.

“It’s scary stuff, she said. “I think about my girls all the time.” But she considers herself lucky that for now, at least, she is living as a breast cancer survivor.

And the walking? “I can do it,” she said. “There are so many people that can’t do it. My aunt couldn’t do it....And I’m going to raise as much money as I can, because I don’t know if I can do this again. I don’t know if I’ll be able to. Who knows?”

The Avon Walk for Breast Cancer will be held in Boston on May 15 and 16. For more information or to sponsor Susan Larsen, visit her link at the Avon Walk Web site at avonwalk.org/goto/susan.larsen.