(CNN) -- When Chris Dolley's sister-in-law asked her to join her in a weight-loss quest, the fast-food fanatic thought to herself she'd do it to help her relative out. It really wasn't about personal goals or missions or health.
After all, she thought she'd probably last only a few weeks and end up back at her original weight, while her skinny sister-in-law went back to her pre-baby mark on the scale.
Well, a funny thing happened on the way to failure.
It worked this time. It took a while -- several years -- and it wasn't easy, but then again, it wasn't that hard either, she says.
Dolley, a 40-year-old financial assistant from Wyoming, Michigan, had tried lots of diets and none of them made a difference. But working with her sister-in-law, who had just given birth to a boy in 2005, she made sudden and drastic changes to her diet and started to exercise.
Now, she's 102 pounds lighter, down from 229, and has run 20 road races, including a recent half-marathon. She no longer eats fast food. If you see her at the drive-thru, she's just getting a Diet Coke, she says.
Background: Dolley says she had a traumatic childhood, one that included sexual abuse in her pre-teen years. To try to conquer the anguish, she ate. She ate a lot.
"You know when you look in the mirror that you are heavy, but you never think of yourself as that heavy," she says.
As the years progressed, so did her caloric intake. She loved McDonald's and Wendy's. She'd even go out of her way to eat there. At her heaviest, she was a size 20.
The moment: One day at work, her sister-in-law, Jodi Yenchar, asked her to join her as she tried to lose the weight from her recent pregnancy. '"I thought, 'OK, whatever,' " Dolley says.
So they started a food plan that limited Dolley to 1,200 calories a day. She thought there was no way she'd be able to lose weight.
"I have no willpower," she says. "Well, at least I didn't."
One week after she started she stepped on the scale and was surprised to see she had lost 5 pounds. That had never happened on any of her previous diet attempts. She stuck with the program, encouraged by the fact that she was needing to buy new clothes.
How much she lost: 102 pounds. In May 2005, she weighed 229 pounds. These days the 5-foot-2 Dolley weighs 127, a weight she feels pretty comfortable at. She's now a size 2.
How she lost it: She cut out the fast food trips and along with her husband, Mike, began to watch how much sugar and processed foods she ate. For exercise she started walking.
After she lost 30 pounds, Dolley joined a gym and changed to a run-walk program. It included several minutes of walking as a warm-up, then one minute of jogging, then a recovery walk of a few minutes, then more running. The interval workout would last 20-30 minutes.
After a few months she went to run outside to train for a 5K (3.1 miles) race.
"And I hated it," she says, "It was hard. I was tired. I didn't like it -- mostly because it was hard." But it became easier and by the day of the 5K, she had worked her way up to running two miles without slowing to walk.
How she's keeping it off: Dolley is a calorie counter. She uses a computer program to log in advance what she is going to eat. Each night she plans out the meals for the next day.
She says that while the wafting odor of fries still tempts her when she drives past a McDonald's, she focuses on the fact that there are so many calories in that red box.
"If I am going to eat those [number] of calories, I am going to get something out of it," she says.
She loves the sweetness of oranges or eating a banana. Almost every meal includes a salad.
She doesn't even really spoil herself after a long race. Actually, she did once, plowing through a "big ol' hamburger" but she quickly had gastronomic regrets.
She would rather go to the market to get fresh and local fruits and vegetables than slide through the drive-thru. She says her last fast food meal was at least two years ago.
"Yeah it tastes good, but if people saw what it was doing to their bodies," she says.
She keeps reminders of her old self to make sure she stays headed on the right path. One is a picture that she keeps on the refrigerator. The other is a pair of pants -- some "fat pants'' -- she puts on every now and then to remind herself of her former largeness.
Her advice to others: Be willing to make some changes, even small changes to your diet will help. If you drink soda, switch to diet soda. If you eat fast food, gradually cut back on the number of times you eat it each week until you cut it out. Cut out excess sugar.
Dolley also recommends the run-walk program to friends who approach her for advice. There are a bunch out there; she recommends the Couch to 5K program.
She also says dieters expect to feel some self-doubt.
"There were a couple of times when I felt a little defeated and I wasn't sure I wanted to do it anymore," she says. "And I thought, 'Everyone else is eating cake, I want to eat cake.' But as I kept dropping weight, and I kept dropping sizes, I kept having fun going out to buy new clothes."
People have taken years to convince themselves they can't lose weight, she says. The big first step on the race to fitness is to finally realize it's not as hard as you imagine it to be.