She was preparing for this year’s grueling New York City Marathon when she found herself in the fight of her life.

Elizabeth Marks, a 59-year-old Upper West Side real-estate broker, recently recounted to The Post how she was readying to go out on a 20-mile trek in August to train for the marathon when “I was putting on my running bra, and I felt this lump.

“And it was like that instant dread,” the married mom said.

“I tried to convince myself it was a cyst and it’s nothing,” she said.

She even went on the run.

But a few days later, she went to the doctor for tests and soon learned the 3-centimeter lump was a malignant tumor and that she had stage 2 breast cancer.

“I just had this overwhelming sadness,’’ Marks said.

Soon, she was in surgery for a double mastectomy and breast reconstruction. Marks said the fact that she was putting on a snug sports bra that day may have helped save her.

“Thank god I was running that morning, because if I hadn’t felt [the lump], I probably wouldn’t have had my mammogram until a month or two later, and who knows how much further along it would’ve been by then,’’ she said.

But she overwhelmingly credits her doctors, Dr. Elisa Port, Mt. Sinai Hospital’s chief of breast surgery, and Dr. Stafford Broumand, a plastic surgeon who specializes in breast reconstruction at 740 Park Plastic Surgery, with really saving her.

Marks, who had her surgery last month, noted that October is breast-cancer-awareness month — and that not everyone has the options she did.

About 2 million people get diagnosed with breast cancer every year, and roughly a half-million annually die of the disease.

But in terms of breast reconstruction, while the federal Women’s Health and Cancer Rights Act of 1998 requires some insurers to pay for it after a mastectomy, the law doesn’t apply to Medicaid, meaning coverage will differ from state to state.

In New York, state law requires general insurers that provide medical and surgical coverage to pay for reconstruction, but self-insured health plans don’t have to foot the bill, according to the state Department of Health.

“There are people that don’t have access to things that I do here in New York and I’m very fortunate,” said Marks, who has a 22-year-old son.

“I’m really, really fortunate that I could make those decisions, and I’m very grateful for that.”

She added that she is already back hitting the pavement again — planning to compete in next year’s marathon