Diana Smith has struggled all her life with her weight.

Like so many other Americans, the Springville, Utah, woman would work at it, lose some weight, and then gain it all back again.

Everything changed when her younger brother came home from spending a year in Africa on a church mission, where he lost 100 pounds.

@livylovestorun/Instagram

"Last year, when he got home, I saw him and I was shocked," Smith, 25, told ABCNews.com.

Her brother didn't set out to lose weight, said Smith, a married mother of two. He dropped the weight simply by walking a lot and eating less. So she began to do the same.

She'd take her young children - her daughter, Olivia, now 3 years old, and her son, Harrison, now 2 years old - for long walks every day. Eventually, she started running.

Her Motivation

Twelve months later, Smith has gone from weighing 285 to 185 pounds. She has run half-marathons and is gearing up for her first full marathon.

"I have so much energy," she said.

"It's a little surreal," she said of her weight loss. "My mind hasn't really caught up with me yet … but I feel healthy now, so that was the most important thing for me."

One of the main motivators for her dedication was her desire to have another child eventually.

"[For] my second child, I was a high risk pregnancy because of high blood pressure … and just [to] know that I can go to the doctor and not be ashamed of my weight, that's worth it all by itself," she said.

In addition to increasing her physical activity, Smith - a trained graphic designer who now stays at home with her children - has also made major changes in her diet.

"I started to use the app My Fitness Pal, which tracks my calories. Basically, I cut out fast food, soda, just … common sense things I knew weren't good for me," she said.

And she's not finished.

"I'm 185 [pounds] now and I'm 5 [foot] 7, so I want to be in, maybe, the 150s or 160s," she said. "That's my healthy range. … I know I want to get lower than what I'm at right now."

Dealing With Jealousy

She said she has also been motivated by the encouragement she gets on Instagram. She has more than 23,000 followers, and gets a flood of praise with each new picture posted to her account, LivyLovestoRun.

"I've had major, major applause, congratulations, just from so many people I've never even met," she said. "It's been amazing. That's been one of the motivators to keep going, because I realize, like, people need to see that even though I'm just a mom that doesn't really understand nutrition and never [has] really been trained … that I can lose this weight. And it's really helped me help others see that they can do it."

But not everyone has been so effusive in their praise.

"I can tell [that] people that I used to be friends with they don't- You know, they're jealous," she said, adding that she's not focusing on the negative.

Smith kept a blog where she documented her weight loss, exercise and diet staples.

She's made such a turnaround in her life that she now finds it strange when she isn't able to keep to her new routine.

"I was at Disney World last week and so, like, my whole life was kind of different because I was on vacation," she said. "I ate whatever I wanted and it was really hard."

There's No Quick Fix

She had a eureka moment after several people asked her to share her weight-loss secret.

"For a while, I kept getting confused why people kept asking me, 'What are you doing? What's your secret?'" she said.

She'd simply reply: "There is no secret. You just have track what you're eating and make sure you're not overeating."

"But then I thought, 'Oh, you know what? That's how I used to think,'" she said. "I used to think that everything was a quick fix … and it's not that way. It's just hard work."

When she hit her 100-pound weight-loss mark, Smith became less strict on her food intake but still adhered to her routine.

"I still count my calories," she said. "I still run every single day or at least try to get to the gym, and I think that I'll always do that because now my body craves it. … I can't not do it."

Having the support of her husband, Daniel, has allowed her to fulfill those objectives, she said.

"Obviously, he loves the way I look. … He's just really supportive," she said. "He's always willing. When I go run, you know, it takes several hours. He's always there to watch the kids for me, and he's always saying, 'No, you need to sign up for that half-marathon. You want to do it, just do it.' So yeah, it's great."