A clear April day, pre-Covid, and I find myself among a throng of runners halfway round the Brighton Marathon 10k. It is not going well. My feet are heavy and my lungs tight. Breathless, I grind to a halt and runner after runner starts to overtake me.

What the hell am I doing? This is just too hard. I scoop off my baseball cap and feel the merciful breeze on my bald head and the pulse in my puffy red cheeks. The fact is I’m full of chemo drugs. I should be resting, not doing this stupid race. I want to collapse on the floor and cry.

The worst of it is that I am going to let people down. If I stop now, I will disappoint my new friends — the ladies in the Cancer Running Club who have kept me sane and even cheerful throughout some of the worst months of my life.

I had never expected to meet any of them, of course, still less that they would become so hugely important to me.

It was in December 2016 that I received a letter inviting me for a voluntary mammogram as part of a pilot scheme to test women under 50.

I was 47, happily married, with three beautiful daughters, living by the sea and loving my life as an author, and I almost didn’t go to the appointment. I was frazzled, with a vast pre-Christmas to-do list, and a mammogram seemed like the last thing I had time for.

Except — there was something niggling at the back of my mind. Earlier that year, I had noticed a tiny dimple at the bottom of my left breast. I had been to the doctor about it but he had said there was nothing to worry about. The mammogram would set my mind at rest, I told myself.

The nurse at the clinic didn’t reassure me but she couched her concern in gentle terms. I was likely to be called back because of that dimple, she said, but in nine out of ten cases it was absolutely fine. I was to go away and enjoy Christmas.

During my follow-up appointment, the atmosphere turned very different very quickly. As a nurse pushed a box of tissues towards me, it dawned on me that this was ‘the quiet room’ and I was being told bad news.

A biopsy swiftly followed, and when I begged the radiologist to tell me what the dark mass on the screen meant, she confirmed my worst fear.

Over the following week, before I knew any hard facts, I convinced myself I was dying. Unable to sleep, I wrote goodbye letters to my children and husband and sobbed into my pillow for the future life I had lost.

It was actually a relief to hear my post-biopsy diagnosis seven long days later — grade two lobular breast cancer — but a shock to be told I was facing an imminent mastectomy, followed by six rounds of chemotherapy, then a month of radiotherapy and five years of the hormone drug tamoxifen. It was curable but it sounded like a very long haul.

Telling my children — Tallulah, then 17, Roxie, 14, and Minty, ten — was the hardest thing I have ever had to do.

We called our eldest into the sitting room and my husband, Emlyn, gently explained that we had not just been to an author event, as we had said we had, but had just come back from a clinic.

She looked at me, her eyes wide. ‘Oh my god, Mum,’ she said. ‘You’re not pregnant, are you?’ I burst out laughing but, although this broke the ice, it was still devastating to tell her the truth.

I’m a glass half-full kind of girl, but it was almost impossible to stay positive when, over the following weeks, everyone suddenly started treating me differently.

It felt as if a big label had been slapped on me. ‘Have you heard about Josie?’ I imagined everyone saying. ‘She’s got cancer.’

Everything that previously defined me seemed to be swept away in one go. A week later, I was approached at the school gate by Ros, a fellow mum, who told me about a fitness charity initiative of the Brighton & Hove Albion football club.

Fitness? I wanted to crawl under my duvet and stay there —but she cajoled me out on to the seafront to meet a special group of runners all of whom had been affected by cancer and were in different stages of treatment.

That first meeting was a shock. I felt like a heavily pregnant woman in the midst of new mums, hearing all the horror stories for the first time.

I was soon told about operations, embarrassing hair loss, scary infections and other grisly things potentially coming my way. And yet there was a comforting British gallows humour amidst it all.

I liked every one of them. There was outspoken civil servant Jane, a tattoo-covered midlife goth, who was just ahead of me in treatment terms and already losing her hair after chemo. Down-to-earth Paula, who worked with disadvantaged teens; driving instructor Maddie, full of wisdom; and singing teacher and political activist Birgit. There was gentle Hannah, too, who was struggling to find her mojo now that her treatment was over.

Together, they all were strong, irreverent and, well, normal women.

And the running? It was slow but it didn’t matter. Just the act of being outside, laughing with my new gang, felt like I was putting two fingers up to cancer and reclaiming my identity.

Quickly, I began to look forward to my weekly meetings with my new tribe. It was a lifeline.

As I approached surgery at the end of January, I took solace in their understanding of my fear, in how reassuring and practical they were.

And as treatment went on, their support was crucial — texting me before my first chemotherapy session, joking about my various ridiculous looks, lending me wigs and scarves when my hair fell out.

One day, a month or so after first meeting them all, I arrived for our run only to see a reporter from the local paper.

‘It’s so great, you’re doing the Brighton Marathon 10k,’ she said. ‘The what?’ I asked Ros, horrified. ‘Oh,’ she replied, with a grin. ‘Didn’t I mention that this was a training group?’

I was freaked out by the prospect, but if the other girls could do it, then so could I, right?

Well, no, as it turned out — or that’s what I thought on that April day, halfway round the 10k course, sweating and feeling defeated, with the rest of our team well ahead of me.

The week before, I had been in hospital with a dangerous case of sepsis, but I had been determined to run.

Now, it was starting to look like that was the wrong call. It was just so tough — I was bitterly disappointed.

But then, as I stood there breathing heavily, the most wonderful thing happened.

Runner after runner tapped me on the shoulder. ‘It’s OK. Keep on going,’ one called. ‘I’m clear five years.’

‘I’m clear ten years,’ another lady said. ‘You can do it.’

On and on the messages came and my feet started to move. Could I catch up with the others? Might it be possible I could really do this?

I stumbled on and then noticed a runner who had fallen into step beside me. She was the picture of good health and, to my surprise, told me that she’d had stage four cancer but had run all the way through her treatment and had subsequently made a full recovery. She told me to keep going. I will, I told myself. I can do it.

She pulled away and I thought that was the last I would see of her — but on the last turn of the race, my friendly runner waved at me.

She was waiting for me, she explained, because she wanted to tell me something. At her lowest ebb, in the middle of her treatment, a woman in a cafe had approached her.

The woman explained that she knew exactly what the runner was going through and had taken off a little silver butterfly pendant she was wearing. Giving it to my friendly runner, the woman in the cafe said it was a butterfly of hope, to keep the faith that life would be better than ever on the other side of cancer.

The runner told me she had been wearing the necklace for three years but now it was time to pass it on. We stopped and she took off the necklace and put it around my neck, saying it was now my butterfly of hope. In time I could pass it on myself but, for now, it would help me stay positive. We had a hug and a few tears and then she was off. I never even knew her name.

I was so moved by this act of kindness, and so touched by the solidarity of all the amazing women in the race, that I felt I had to write about it, once my treatment was over.

But there was still a long way to go. It wasn’t until the end of July that I finished all the appointments and the doctors gave me the all-clear. The long road to recovery could begin — helped by the running gang and our monthly curry night and the WhatsApp banter.

They were the first to cheer me on when I announced I was writing a novel called The Cancer Ladies’ Running Club — because they, like me, feel strongly that we need to talk about our experiences.

With one in eight women getting breast cancer, it’s a subject that affects many of us and yet seldom do we hear a positive tale. The truth is, with the excellent treatments we have nowadays, many women like us are not just surviving but thriving.

I’ve dedicated the book to my running pals, of course, though it is very much a work of fiction.

It is story about how, sometimes, all it takes to turn your world around is putting on a pair of trainers and getting out into the sunshine. It’s a story about the power of positivity and how, so often, it’s our friends that help us find our feet.

Josie Lloyd’s novel inspired by her experiences, The Cancer Ladies’ Running Club (£8.99, HQ), is out on Thursday.