Would the Grit Doctor's advice to release her inner bitch help Emma Bartley to get her butt off the sofa? Could she learn to enjoy a tough love exercise regime? The first step with this regime is a trip to the bookshop

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“Who is the Grit Doctor?” asks the first page of Run Fat B!tch Run before answering its own question. The Grit Doctor is Ruth Field’s “inner bitch”, the voice that shouts at her to get up from the sofa.

It’s probably a useful idea, if a little offputting for this reviewer, making me think of stage personas such as Mariah Carey’s “Mimi” (as in “The Emancipation of Mimi”). Why is it that for a woman to be assertive, to take time for herself and make it evident that she cares about her appearance, she has to put the responsibility on to an imaginary friend? Those qualities are not, actually, bitchy.

But you didn’t click on to this article to hear me bore on about feminism any so let’s proceed to a more important question. Is this book going to make you slimmer? Is it going to make you want to run?

In fact I think that this book is going to make you slimmer. It is going to make you want to run. And that's no small deal. Field is frank about the downsides of running: it is boring, repetitive, relentless, punishing. However, as she says: “There is no magic pill… life isn’t easy." Instead, she promises to get you enjoying hard. Embracing hard. There are other things about hard but I’m struggling to keep a lid on my innuendo reflex.

Run Fat B!tch Run  is actually a very short book divided into three parts, then divided into subsections, which

are mostly written as sort-of bullet points with motivational comments from TGD. The first section tells you to put your shoes on and go outside, walking a loop, gradually building up to running the whole thing – starting by jogging for only five minutes of it. The second focuses on diet and eating habits, sensibly banning fizzy drinks, sweets, cakes/biscuits/crisps (weep) and takeaways. One subsection entitled “The Golden Rules” is two pages with only three words on each with the text in huge capitals: DRINK MORE WATER, and EAT LESS CRAP. In section three, “Now what?”, these make up two of the author’s three-point summary of her book, the first being “run”.

As you may have gathered, Run Fat B!tch Run is not rocket science – but nor does it claim to be. Field, who narrates short segments of the experience that got her running – essentially being a very competitive but at the time slightly overweight person whose friend stood up in the pub and announced that she would be doing the London marathon – is simply pointing out what we all know. If you want to be thinner, you just have to move more and eat less.

So why do you need the book? Perhaps you don’t – if the book was suggested to TGD you suspect that she would ask why you were sitting around and reading it and not running around your nearest green space. But for those who aren’t fit, who are perhaps a little bit or even a lot fat, the value lies in its frankness. At times, yes, this is heavily laboured – “Bullshit!” is a representative comment – but with so many kind lies around us, from the public outrage at Karl Lagerfeld saying that Adele was overweight to the friend who suggests going for a milkshake instead of the run you have scheduled together because “we’ve earned it”, that the truth is quite refreshing.

Now what? In the final part of the book Field gives the benefit of her experience, adding in running buddies, 10K training plans, push-ups and pull-ups and some useful appendices covering online tools, a running log and medical advice. But I sense that she herself would not want too much emphasis to be on these parts before we can run. Before that, we should walk. And if you really can’t bear the idea of a walk around the park, a walk to the bookshop to find out why is a decent start.

Run Fat B!tch Run (Little, Brown, £10.99) is out now