When Marie Lisette Flores sets a goal for herself, she sees it through to the end. And then she moves to the next challenge.

Take her decision in 2005 to join a 75-mile bike ride from Miami to Key Largo to raise money for the National Multiple Sclerosis Society. She wasn’t particularly athletic, but she trained for three days and set off.

Fast-forward to 2009 when a friend told her about a triathlon to raise money for cancer. Flores had never participated in a triathlon before but signed up immediately, thinking about her daughter, a thyroid cancer survivor. She fell in love with the race and signed up for four more that year.

Then there was the time in 2010 when she joined a three-day, 375-mile group bike ride around Puerto Rico with her husband. And earlier this month, she ran what’s called Goofy’s Challenge at Walt Disney World: a full marathon and a half marathon in a single weekend.

“I don’t know what’s going to happen to me tomorrow, so today I do all of this because I can,” said Flores, 47. “I feel a positive energy when I exercise, so I want to keep doing it.”

And that’s why she signed up to run 26.2 miles in today’s ING Miami Marathon, joining some 25,000 runners. It will be her fourth marathon.

What makes these accomplishments even more exceptional is that Flores has multiple sclerosis. She was diagnosed with the autoimmune disease in early 1999. Among her first symptoms: the left side of her body went numb for several days and, later, her right arm suddenly became so weak she could no longer raise it.

For a year after her diagnosis, Flores took vitamins and changed her diet, eliminating red meats and fast-food. Her doctor told her to eat well, exercise and shy away from the sun, because too much heat can trigger or worsen attacks. And so Flores would go on occasional walks in her Kendall neighborhood after the sun went down.

A year passed without another episode. Flores convinced herself that she was fine, that she didn’t have MS and that she could eat whatever she wanted.

So Flores, who was born in Puerto Rico and grew up in the Dominican Republic, went back to a diet of Big Macs, fries and chuletas, or pork chops. In August 2001, while on a Caribbean cruise for her daughter Kristell’s 15th birthday, Flores basked under the sunlight.

“I exposed myself to the sun. It was a beautiful week,” she said. “But I felt the heat.”

Days after returning to Miami, Flores’ right arm went completely dead. It was her third episode, and the first time she needed hospitalization.

Finally, Flores accepted the reality: she had MS and needed to take better care of herself.

In the next few years, Flores would take part in a blind study for a new medicine, natalizumab (known by its brand name Tysabri). Once a month, Flores got a monthly infusion of what she now knows was a placebo, and not the drug.

Meanwhile, the MS episodes grew increasingly severe. After her last hospitalization in 2005, she could barely walk without a cane.

By that time, the study was over and the drug was available on the market. Flores, like others who participated in the study, received a year’s supply for free. She hasn’t had another episode since.

“She just took off and never looked back,” said Dr. William Sheremata, her longtime neurologist. “She was only able to walk a short distance before, and now she’s become this super athlete.”