The harsh words you have to hear to get motivated

It's much easier to find the motivation to chill out with a glass of red wine or take a bubble bath than it is to pull on your sneakers and work up a sweat. That's where The Grit Doctor comes in.

"I am a ruthless, no-nonsense motivator who will force you to do the things you don't want to do," Ruth Field writes as her alter ego, The Grit Doctor, in her first book, Run, Fat B!tch, Run!

"I will whisper in your ear 'YOU FAT BITCH' when you are contemplating one more roast potato. I am the voice you need to listen to and obey."

Field says this brutal method of motivation works for her. And the constant ranting inspired her lethargic husband to get out and run around the Parisian countryside where they used to live.

"My husband was fed up with me nagging him to run, so he said to me, why don't you write down what you have to say about running because you're very passionate about it."

That's how the book Run, Fat B!tch, Run, came in to fruition. Although Field never expected it to be be published and was pregnant when she wrote it, so says her tongue may be more harsh than intended.

"I've often thought that maybe other people might have more success [with exercise] if they adopted the same, slightly brutal approach."

A few years ago, Field was what she dubs a "slow coach". She was in an "exercise rut". But her inner Grit Doctor forced her to get active. She set goals, came up with a set running track and picked up the pace. She now runs about 5km, three times a week, mostly because it allows her to eat more. And while she'd rather play tennis, Field says running is a far more efficient form of physical activity - it's high intensity, doesn't require lots of equipment and can be done anywhere.

In the opening of her recently published book, Field asks readers to sign a "pointless pledge" declaring "I am a FAT BITCH and I want to do something about it ... I will haul my fat arse off the sofa and get out the front door."

This might seem harsh, but Field says it's all done jest.

"When you read the book, you realise it is me speaking to myself, so I'm not calling everyone else a Fat Bitch ... it's what I say to myself to get myself out of the door.

"The way in which to take it is very much with a massive smile and a sense of humour.

"It's not about beating yourself up, it's about getting yourself out of the door to run, because that's the really hard thing.

"Yes, it is confronting, yes it is a bit mean, but try it out and see if it works."

Run Fat Bitch, Run! by Ruth Field, published by Sphere, is available now. RRP $37.