(CNN) -- If someone had asked Kelly Pless to describe herself three years ago, the word "fit" would have never crossed her mind.
Kelly Pless weighed 220 pounds at her heaviest. She lost 95 pounds through diet and exercise.
For most of her adult life, the 31-year-old graduate student from Sunny Isles Beach, Florida, has struggled with her weight. She started gaining as a teenager and by the time she graduated from high school, she was carrying 215 pounds on her 5' 2" frame.
Prom, she says, was a nightmare.
"I had to go to three different stores to buy a dress," Pless said. "I had to buy the biggest, also the ugliest, prom dress the store had because it was the only one that would fit."
After high school, she lost 50 pounds. But because she hadn't done it in a healthy way, her weight crept back up to 220 pounds.
At 28, she started having trouble breathing and doctors told her the weight was to blame. She reached her breaking point.
"I remember being heavy and feeling like being fit just wasn't something I could be," said Pless. "I remember feeling like even if I tried, it wasn't something my body was capable of." Kelly Pless talks about her incredible journey »
Despite her doubts, Pless decided to do something. Fortunately she didn't have to look far for inspiration. See before and after weight loss photos from CNN.com I-Reporters »
"My manager at the Kennedy Space Center ran marathons, and he was the same age as my father," she said. Because her own father had diabetes and was in poor health, he seemed much older, she said.
Over the next three to four months, she began walking, without any real goal or expectation. Pless believed that if she just focused on eating less and moving more, everything would fall into place.
"At first, it was hard to start exercising because I was worried people would make fun of me," Pless said. "But then I just told myself, if that's the worst that could happen ... I just got out there and didn't care."
Eventually, she started to run or "shuffle" as she jokingly recalls. She also adopted an "eat to live" philosophy and satisfied her cravings for sweets by eating lots of fruit.
"I changed how I felt about food and what it meant to me," said Pless, who occasionally indulges in a bite of birthday cake or a piece of chocolate.
"One of the first things I cut out was cakes and cookies. That was my weak spot. After a few months of cutting those things out, I focused more on portion control," said Pless. "I pretty much eat when I'm hungry and don't eat when I'm not and really try to pay attention to when those times are. Make sure I'm not eating out of boredom or [at] social events, I try to make sure I'm not overeating, just because everyone else is." Kelly Pless shares her weight loss secrets »
Pless pays close attention to societal pressure, which she believes is the reason many people overeat. Restaurant servings are about three times bigger than a normal portion size, she says. She makes sure she doesn't overeat when dining out simply because the food is there.
"What's hard is to change how you feel about foods that you love or that aren't necessarily good for you, or actually change how you look at food. That was the hardest part for me."
Instead of giving in to the temptation or convenience of calorie-laden or fatty foods such as cheeseburgers from the drive-through, Pless asks herself, "What do I really want to eat? Or, what does my body really want right now?"
All of the hard work and determination paid off. Pless has lost 95 pounds and kept it off for 1½ years. As a result, she says, she's healthier and more confident.
She's also set a new professional goal -- to pursue a doctorate in food and exercise psychology -- so she can help others who are battling obesity and eating disorders.
"[The] negative side to weight loss is that people treat people differently. Being fat was a good filter -- I'm automatically treated better by people because I'm thinner. Society is so hard on people who are overweight or obese," said Pless. "Now, those people think I'm funnier or smarter."
Pless runs about 40 miles a week while she trains for two marathons she plans to run this winter. The first is in November in Las Vegas, Nevada, and the second is in Miami, Florida, in January.
Forty pounds ago, the first thing she wanted to do once she lost the weight was to have a tummy tuck to remove all of the loose skin. But now, she said she can't imagine taking the break from running that recovering from surgery would require.
"Running has become a constant for me and does so much more for me than maintain my weight, which is now about 125 pounds," said Pless.
If her past is any indication of her future success, Pless will certainly cross the finish line.
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