On Sunday, the 40-year-old mother of one will be taking part in her fourth marathon -- in New York -- "a dream come true."
The professional life coach began her running career began in 2010, when she took part in a short race and came last.
"When I got to the finish line, there was no finish line left, everybody had left and gone home," she tells AFP, dressed in black leggings and a pink hoodie, her blonde hair in a pony tail after a quick jog in Central Park.
She remembers feeling "very embarrassed and thinking maybe I shouldn't be a runner, maybe I am not welcome in the running world." So she decided to inspire other women of a similar shape.
"That's when I started writing the blog," called "Too Fat to Run," she says.
The London Olympics later inspired her to run her first marathon.
"I've done maybe 30 half marathons and ultra marathons and triathlons... anything that kind of challenges me," she says.
"For me it's about inspiring women of all shapes and sizes to give it a go," she says.
"I feel quite passionate about larger women running, because we weren't very visible in the running world. When you think runner, you think tall, slim, fast, and lots of people want to run for lots of other reasons."
Creffield, who says she does not know how much she weighs, wears a dress size 18 in Britain, which is a 16 in the United States.
- 'Heckled a lot' -
It is in Central Park, where the fall leaves are already turning yellow, that Creffield hopes to cross the finish line along with the more than 50,000 other runners taking part from around the world.
The average time to complete the New York marathon, which takes in the city's five boroughs, is four hours, 35 minutes.
Last year's winner, Geoffrey Kamworor of Kenya, romped home in two hours, 10 minutes, 53 seconds.
Creffield calculates that it'll take her between six and a half, and seven and a half hours. She'll run as much as she can, and walk when she needs a breather.
People take up running for all kinds of reasons, she says, not only to lose weight or get in shape, but to counter depression, to socialize or even carve out a little me time.
Although she has been heckled and insulted for running, Creffield stops at nothing and is confident that little by little, attitudes are changing.
"I've had things thrown from car windows at me when I train. I got heckled a lot of the time. It tends to be either children or grown men. Sometimes people shout things they think are motivating, but they are not," she says.
"Because you're big, they think you're new to running and they shout things like 'it gets easier!' and I'm like 'no it doesn't, I've been running for 15 years and this is my pace,'" Creffield smiles.
"There are so many more things that define us as women than what we look like."
Although some people want marathons reserved only for elite runners, a growing majority -- especially in New York -- believe they should be open to as wide a pool of people as possible.
"Our mission at New York Road Runners is to help and inspire people through running, regardless of their age or ability. Everyone is welcome," explains Chris Weiller, spokesman for the group that organizes the marathon.