In my case, it was a book. Ruth Field's (pictured above) Run Fat B!tch Run arrived just before Christmas. The pledge, above, is on one of the very first pages. It was exactly how I felt. Exactly. I devoured the book (could have been worse, it could have been chocolate I devoured) while my children had their swimming training (could have been better, I could have been swimming myself). Field got it. She got me. The only reason my arse is fat is because I choose to sit on it.

"You just have to start doing it," Field says, on the phone from London. I'm her first media interview for the book and this quiet, 36-year-old mother of 16-month-old twins still can't believe anyone is interested in what she has to say. Our interview takes place before Christmas (more on the delay in the actual writing of this story later – and no, it's not because I am a lazy bitch too), and since then the book, and the subsequent blog, Facebook page and Twitter feed have taken the world by storm. Field has touched a nerve.

Her premise is that we need to take a long, hard look at ourselves, stop making excuses and just do it, to borrow the Nike phrase. It's nothing new. But what makes her believable, what makes her program seem achievable, is that she realises that the simple premise is not that easy. She's the first to admit that since having the twins she's found it much more difficult to find time for running.

"It's been much harder to get out and run since having the babies, compared to when I was without children," she says.

"It's much harder to motivate yourself when you have children because you're knackered all the time. I had no idea how much hard work motherhood would be. It's about 20 times harder to get myself out the front door, I'm just so tired. I want to collapse on the sofa but once I do get out I'm delighted that I have done it."

She says we can be our own worst enemies in that respect. We get saddled by life and all of the stuff we have to get done, and exercise takes a back seat. Ultimately it boils down to choice. Some people choose to do the exercise and others don't.

"I just wish people were more honest about that, rather than having this conversation which is based on all of the excuses, all of the reasons we don't do what we all know deep down we should be doing, which is getting out, moving around and eating less rubbish."

The idea for the book first came up while Field and her husband Ollie were on their honeymoon, visiting Ollie's grandmother in Perth. "While we were there we decided we would jack in our jobs back in London, move to France and Ollie would write a novel and I would do some painting," she says.

"Ollie's grandmother was so encouraging. She said, 'Take the time off and do it'.

"Australia gave us this sense of possibility, so we took our chances and that was it."

The couple did move to France, Ollie started his novel, but Field, who was pregnant by this time, was living, she admits, a life of frustration.

"I didn't have enough to do, my painting was crap, I was trying to learn French, I was crap at that too, Ollie wasn't writing as much as he could have been and I was nagging him to run, thinking that that might help him be more productive.

"He did start running in France and was much more productive, and then one day he said [I] should start writing some of what I was telling him down.

"That's how the book started, me documenting Ollie's running program, his diminishing belly. It was a fun project. I didn't think it would become anything. Then once I'd written out all this stuff, a few people said it was quite good and I should let someone have a look at it before the babies came."

Field is quite self-deprecating – crap at this, crap at that. That's where The Grit Doctor comes in – her alter-ego, the "ruthless, no-nonsense motivator who will force you to do things you don't want to do". It's the Doctor who whispers in your ear "YOU FAT BITCH" when you are contemplating one more roast potato.

A little harsh, perhaps?

"Sometimes we're just too kind to ourselves," Field says. "All this namby-pamby, feel-good crap we feed ourselves all the time. I think it's demotivating, saying 'I'm so great, I'm this, I'm that'. Maybe it's okay if it makes you feel good, that's not a bad thing, but at the same time, if it stops you getting stuff done, or changing things you want to change, then it's not a good thing. If I really want to do some exercise, or get out there and run, and I haven't for a week, the way I get myself to do it is to be a bit hard on myself. I find it quite motivating. I don't know if it's a terribly healthy way of doing things but it works for me. If you can laugh about it then it can't be unhealthy."

There's a program in the book to get you running, steps to follow that will have you running five kilometres in eight weeks. It's based on a system Field did with her mother, 50-something at the time, who saw how good running made her daughter feel.

"I was living at home at the time, trying to pay off my last bit of student debt, at 30 years old if you can believe it. I thought if I moved out of London and didn't pay rent for a year, I might get it all paid off. So, in true Grit Doctor fashion, I moved home. I was running a lot then, I was down in the dumps, I was missing London, I didn't have a boyfriend, I was in debt. I was probably running four to five times a week.

"My mum was sort of impressed and a little bit terrified by my levels of motivation, but she soon saw how much happier I was after I'd been running. So she said, 'Do you think I could do it?' "

Field recalls how chuffed she was.

"I thought this would be great for mum. She was in her 50s, she'd been super fit when she was a youngster, but like most women – most might be a little unfair actually, but most of the mums I know in their 50s – don't run or do much exercise. She was overweight and unfit. On the first day we just walked a lap of the local park near home in Ascot – it goes around a lake, a really beautiful lake – and then we built up from there. I'd run and mum would walk, and then she started jogging a tiny bit, then a tiny bit further until she could run the whole way."

So, that's where my delay in producing this article comes in. I thought, if Field's mother could do it, so could I. So I started running. Well, that's a slight exaggeration. It's not like I'm doing it regularly, or even that I followed Field's program. I'm not a runner. But occasionally I have been for a run. Or a walk, really, one that involves a gradually increasing portion of running. And the scariest thing is, as I'm pounding around the fire trails of Bruce Ridge with Rihanna blasting on my iPod, I'm loving it. Loving running. Never thought I'd say that. It's not much, but for me, it's a start. All I need now is a good slap across the back of the head from The Grit Doctor and for her to say, "If you love it so much, why aren't you doing it more often? Get off your arse".

She's got a point.

¦ Run Fat B!tch Run. By Ruth Field. Sphere. 256pp. $29.99. Follow Twitter updates @gritdoctor or visit gritdoctor.wordpress.com/