Scarlett Thomas: ‘Fitness is visceral, earthy, real, sweaty – and new.’ Illustration: Anna Parini

It’s Monday morning and I am running on the seafront. It’s cold and bright, and there aren’t many other people out. I don’t really feel like running, but I didn’t get enough exercise over the weekend, so here I am. I did manage a long workout at the gym on Saturday morning, running 5km and lifting some heavy weights, listening to Snoop Dogg and Dr Dre. Then I went to London, where I drank more red wine than I meant to, and ate a large portion of coq au vin. I stayed overnight, then fretted my way to a grey gym on Wandsworth Road for an eight-hour course on core stability.

Ten years ago, Sundays were for lying in bed with a pack of cigarettes, a cheese sandwich, a copy of Vogue and a novel. But since my late 30s, I have been trying to improve myself. I recently qualified as a fitness instructor, and the course is an add-on. I can’t stop signing up for new courses. In September, I did an online nutrition qualification, blasting through the modules like a kid playing Asteroids. As well as writing novels, I am a lecturer in creative writing, but recently I have not been feeling excited about anything apart from exercise. Fitness is visceral, earthy, real, sweaty – and new.

The problem with core stability is that you don’t move much. You get on a foam roller, and then you get off the foam roller. You stand on wobbly discs and throw things. My Fitbit, a fancy electronic pedometer stuck in a plastic wristband that I’ve been wearing for a few months, tells me that yesterday I did only 12,000 steps, or around six miles (9.5km), despite aiming for 20,000.

Some days I do 25,000 steps, or around 10 miles, collecting them mainly on the tennis court. In drinks breaks, my training partner, C, and I get out our iPhones and compare Fitbit bar-graphs. Have we managed to change the colour on the bar representing the last 15 minutes from yellow to green? “Give me an intense bar!” I shout at him. “Make it green!” I always get more steps than C. One day, he finds me on the treadmill before our practice. “That’s just cheating,” he says.

Am I cheating now, on this cold Monday on the seafront? Maybe, except I can’t seem to get going. My legs are so stiff. I try stretching, squatting, lunging, walking. Each time I try to go from a walk into a run, my shins hurt and my calves seize up. I do one mile like this, then two, but it’s only just above freezing and I need to move more quickly. My new Garmin running watch bulges on my wrist, underneath my three tops, alongside my Fitbit and the holographic bracelet worn to enhance my performance. I have decided not to look at my stats: they are always bad, which makes me try harder, which leads to injury. I’m 42, but this doesn’t matter. If you try hard enough, you can do anything, right? And this is so worth it. After all, sitting is the new smoking,

Last October, I travelled a total of 191.17 miles. I also began having occasional dizzy spells. In November, despite the dizziness, I managed 158.19 miles. But I look at that October figure now – almost 200 miles! – and, after an initial thrill, feel disappointed. Serious runners must do way more. I Google it. Oh. Actually, normal people training for a marathon are advised to run 30-50 miles a week, so I was achieving something. Awesome! Except… Hang on. I wasn’t running all those miles. Some of it was walking to pick up tennis balls. Some of it was walking around bookshops and libraries. Some of it was walking around the house. I close the website. I am a failure, after all.

I long ago gave up on that novelists’ cliche of checking my rankings on Amazon. Novels such as mine sometimes flutter around the top 10 in the week of publication, but then settle into more “cult” chart positions. The thrill of publication, like most things, is over so soon. Who wants to know they are 47,692 in the bestsellers list? I prefer stats I can control.

A recent Fitbit ad shows an attractive young guy deciding to walk rather than take the underground, because by doing so he is more likely to “crush his goals”. A graphic shows him moving ahead of one of his friends, as if life really is just one long fun run. In this world, it is never better to just take the tube. I am not good with most social media, but I do have a few friends on Fitbit, and on the app on my phone I can see how many steps each of them has done that week. I know if my friend V has gone for a run, and that used to prompt me to go for one, too. A friend who works from home is afraid to exercise during the day because her boss follows her on Fitbit; he would know when she was no longer at her desk. It’s not just Fitbit, of course. You can get a Jawbone or a Nike Fuelband or an Adidas miCoach. The new Apple Watch will measure your steps, distance and heart rate, and deliver your emails to your wrist, too. Deep in my soul, I believe all this stuff is stupid, uncool, capitalist, sinister – but that doesn’t stop me. Recently, I have started to worry: do I even exist without my Fitbit? Without data, am I dead?

Scarlett Thomas: “I suppose I did realise, at some point, that I was acting obsessively. But I thought it was fine – after all, it was a healthy obsession.” Photograph: Ed Thompson for the Guardian

It started when a dentist recommended the MyFitnessPal app. He’d lost a lot of weight and I wanted to know how. You can scan stuff, he told me. Barcodes – on sandwiches and salads and everything. Work out how many calories you eat. That was five years ago, and I have MyFitnessPal data for almost all that time. If I want to, I can see what I ate on 2 August 2012.

Like a lot of dedicated trackers, I have connected my MyFitnessPal account to my Fitbit. If I do enough steps in a day, MyFitnessPal “knows” and increases the number of calories I am allowed to eat. I am at the very moderate end of what is becoming known as the quantified self movement – there are people out there who record their temperature several times a day, who regularly send nose swabs to a lab who will track their bacterial profile; but I also cannot imagine getting up in the morning and not weighing myself. One day, when I met C for tennis practice, he was listless and quiet, hardly moving. What was wrong? Was he ill? No, he told me. The battery had run out on his Fitbit and he just didn’t see the point of taking steps he couldn’t record.

If I am honest, I don’t feel too well on the seafront that Monday morning, but I have always exercised through injuries and I’m sure I will be fine. All summer I had a slightly sprained wrist, which I strapped and taped. Last June, I had such a bad allergic reaction to an insect bite that I was almost hospitalised, but I still went to the gym. Rafael Nadal often has parts of his feet anaesthetised before playing big matches, because of his blisters. I persuaded a doctor to prescribe me a cream that would do something similar to my rash, and carried on. The rash turned fluorescent yellow, which freaked me out until I realised it was just tennis-ball fluff that had got stuck.

My Garmin bleeps. Mile three. My legs have loosened and I am running at last. I wonder how fast I’m going. I don’t want to look, but I do, and find that I am possibly running a nine-minute mile. Maybe 8 minutes 50 seconds. Pathetic, but for me that’s amazing. Could I make it eight and a half? I push the pace. Grey waves break gently on the pebble beach to my right. I’d usually have music, but today I left my iPhone – with playlists called things like Training Oct 14 and Weight Room Hip-Hop – in the car; this was supposed to be an easy run, and music makes me go too fast.

Suddenly, there is a group of late-middle-aged people blocking my way. I can’t lose any of this pace; I think I’m running my fastest ever mile. “Excuse me,” I call. “Coming through!” As I pass, I hear them mocking me, or maybe I imagine it, but I give them the finger anyway as I run on, feeling invincible and just a tiny bit aggressive from all the testosterone in my blood. I can hear them imitating my heavy breathing and laughing, but I don’t care.

Deep in my soul, I believe all the gadgets and apps are stupid, uncool, capitalist. But that doesn’t stop me. Scarlett Thomas

Half a minute later, I hear the beep that says I have completed the mile. Just over nine minutes. For me, that’s good. I slow down carefully and start gently jogging back towards my car. I slow to a walk and do a couple of stretches. And then it happens again. I feel dizzy, horribly dizzy. Suddenly there is nothing in my head apart from air, little half-puffs from an unfathomable void. I feel as if I might just float away, float down on to the concrete and… No! God, no. Please, I can’t faint here. Especially not after abusing the only people around for miles. Stay upright, I tell myself. Walk. It feels like trying to start a car that’s run out of petrol. Fine, I tell my body. We will go really slowly. I start to cry. I sort of pray, too, try to think happy thoughts. I realise that one positive thing about this whole experience is that, when I get back to the car, I will have done at least four miles rather than the planned three. I will definitely have more steps than C this week. Maybe I’ll even beat V.

Sometimes, even now, I sit and stare at graphs. Graphs recording my weight loss. Grams of carbs, fat, protein. Steps. Miles. My bathroom scales are connected to the Wi-Fi, which means that each time I weigh myself, a new number is uploaded to the Fitbit website and MyFitnessPal. The graph that represents my weight over the last year is a softly flowing hill: down, down, down roll the numbers. And I think, that is me. The person who used to look forward to a cigarette after a game of badminton and then phone for a greasy takeaway? That is not me. The person who gave up smoking, put on weight, then tried every diet going (except the ones that said you couldn’t eat very much)? She is gone, too. But lately something has gone wrong with the graphs. For the whole of November, I have not lost any weight, despite all my mileage. On my fitness course, I learned that hitting a plateau is a sign of overtraining. But it can also be a sign of being a lazy fat arse. I need to try harder.

I suppose I did realise, at some point over the autumn, that I was acting obsessively. But I thought it was fine – after all, it was a healthy obsession. My favourite days were the ones when I would go to Pilates or the gym, have a quick turnaround and a cup of tea before a two-to three-hour tennis session, then home for a shower and a bowl of complex carbs before going out to play a league match in the evening.

The reason I play through injury is not because I am a good person; it’s simply because I like the feeling of playing so much that I can’t stop. And as a 42-year-old, child-free, partnered woman, where am I supposed to get my kicks? I’m too old for clubbing. I have given up drugs, smoking and sugar. I drink only at weekends. Where else would I get my thrills? I always assumed that endorphins were just a poor man’s high, for people who enjoy drinking tap water and wearing sensible shoes. But here’s a secret: if you get really fit and then really cane it, it feels better than a line of coke.

One day in October, V came round and showed me the heart rate app he’d just got. You put your finger on the little iPhone camera thingy, and it senses your pulse from the light effects of the dilation of your blood vessels. I used to be scared of my pulse, back in the days when I got panic attacks, but not any more. So I downloaded the app. I bragged to V about my heart rate being 55. He said his was 48. I exercised harder. Then, one day, I was on the phone to my father, idly messing around with my app. I was quite stressed, but my heart rate was only 42. Did this mean I had become so fit I just didn’t register stress any more? Was this another little ping of success? My father was diagnosed with lung cancer in July. This was what we were talking about when I realised that my heart was not responding. It was beating, just not very fast.

* * *

What actually happened that day on the seafront? I had some sort of breakdown, I tell people later. Did I pass out? Was I hospitalised? No. After I made it back to my car, I called my partner R, who came to pick me up. The next day I woke up with flu and for a while that’s what I told people: it’s just flu.

I begin a course of antibiotics and go to the doctor about my dizzy spells. Could it be anaemia? Possibly. She books some blood tests, as well as an ECG (electrocardiogram) to monitor my heart. My heart? Yes, she says. It could be that. She tells me to lie down and put my legs up if I feel odd. We visit family, and over the holiday I feel weak, shaky, jittery. I drink wine and try to ignore my mounting anxiety and fatigue, and I spend a lot of time lying down with my legs up. When we get home, I force myself to the gym, but I can’t get through a whole workout. We go for a walk on the seafront, but after 10 minutes I have to lie down on a bench with my legs up while R fetches the car. The next day, I can hardly get out of bed. This is pathetic. I’ve bought my mother and stepfather Fitbits, and I want to inspire them. But I am suddenly not managing more than 300 steps a day. Fitbit friends get in touch to ask what’s wrong with me; they can tell I am ill just from my poor stats.

Scarlett Thomas: “I’d always assumed that endorphins were just a poor man’s high, for people who enjoy drinking tap water and wearing sensible shoes. But here’s a secret: if you get really fit and then really cane it, it feels better than a line of coke.” Illustration: Anna Parini

My blood test results come back: all normal. That only leaves my heart. While I wait for my ECG appointment, I test my pulse regularly, hoping for something like 60 or 70, but often getting 42 or 38. When I get a low reading, I start to panic – even that doesn’t raise my heart rate. I look up athletes’ resting heart rates and find that mine would be exceptional even for an Olympic cyclist.

I try to do yoga. I can’t. I start having extreme anxiety attacks. I read books on ME and adrenal fatigue. My mind still works well enough to write, which probably rules out ME. I don’t have the respiratory infections you are supposed to get with adrenal fatigue. According to Google, I have something called Overtraining Syndrome – a real illness mainly affecting elite athletes, but, increasingly, also normal people, too, which involves exhaustion, lack of results, anxiety and depression. But there hasn’t been much research on it, and when R tells people what is wrong with me, he precedes it with, “I know this sounds a bit bogus, but…”

People keep telling me not to Google my symptoms, but I don’t know how else to find out what is wrong with me. The local GPs don’t know anything about athletes and their problems. Why would they? Online, I can look at studies and reports, and read accounts of other people reading studies and reports. It seems there are a lot of exhausted people out there. Whenever I have strained something in the past, I have found a YouTube clip explaining how to tape it, but there is no tape for this.

My GP wants me to have a 24-hour ECG. I am given a plastic box that smells of fags. I clip it to my jeans and attach wires to my chest. I sense this is not going to be as much fun as the other ways I measure and monitor myself. I have to keep a little diary and record when I have “symptoms”. In the mirror, I look like a broken robot. When I phone for the results, I am told that I have a harmless arrhythmia, probably caused by stress. The really good news is that my lowest heart rate over the course of the day was 55. The arrhythmia was making my pulse seem slower than it actually was. I delete the heart rate app. I do a bit more Googling – it looks like this is definitely Overtraining Syndrome. I wonder what the GP will suggest we do about it. She refers me to a cardiologist, even though I take her a report from the BMJ that says heart arrhythmias in athletes are completely normal.

My heart is a jellyfish on a screen. A sad, soft-looking creature from the deep. Its valves flap like little fins. This image, created by an echocardiogram – an ultrasound of the heart – is not like the diagram I had to learn for the theory part of my fitness instructor course. This is visceral, real, earthy – and it turns out I don’t really want to see the stuff I am made of.

I’ve heard that people get addicted to peeing on sticks and trying to turn them purple. Even I’m not that mad. I think Scarlett Thomas

The cardiologist points out the place where the arrhythmia is happening. It is harmless, and caused by stress. How long have I had it? Oh, well, roughly since the GP suggested I might have a heart problem. So this wasn’t a feature of the original dizzy spells at all? No. I have entered a positive feedback loop. He declares me healthy – and much fitter than he is. But the next day I still feel weak. I start taking more vitamins. I read books about fatigue and try to eat more vegetables. I drink water with salt in it. I meditate.

I go back to the university and start enjoying my teaching again, though I’m still exhausted. One day, I’ll be fine, buzzing with ideas for classes, my new novel. The next, I won’t be able to drive in without a panic attack. All I can do, I realise, is try to manage the bare minimum. Can I sleep for the whole of Easter? I haven’t been to the gym since just after Christmas.

Surely it isn’t over, my lovely, buzzy spell as an athlete? I have not given up on my numbers, although my Fitbit has been lying dead on my bedside table for weeks now. I experiment with different diets and make meticulous notes on the results. I go macrobiotic for a few weeks and then, unconvinced by so much grain, I go low-carb. I examine my macronutrient pie charts. I am thinking of ordering some Ketostix – urine analysis sticks that tell you whether or not you have reached the fat-burning nirvana called ketosis, which low-carbers aim for. But I’ve heard that people get addicted to peeing on them and trying to get them to turn purple. And even I’m not that mad. I don’t think.

One Wednesday, my friend D knocks on my office door. The last time I saw him was at a colleague’s birthday a couple of months ago. I talked a lot about sport and exercise, and an article I’d just been commissioned to write on extreme fitness. It was going to involve doing sledgehammer workouts and battle rope routines. I remember being in the dark, hot pub and just wishing I was on a tennis court or running on the seafront. D talked about his health problems. He used to be a rugby forward; now he is struggling with his weight. We talked about fitness plans, training. Among other things, I suggested he get a Fitbit. He did. He’s already lost half a stone, he says now, just because of that.

I feel excited for him, and enjoy the little secondhand thrill this gives me. How long will it be before I have my own Fitbit graphs again? Smaller ones, perhaps, to show that I have learned something from this experience. More yellow, less green.

Or perhaps I should throw it away, along with all my other tech crap. I should go walking in the mountains in a simple dress with only a packed lunch and a paperback. I yearn for that life, I really do. But there’s a part of me that would always want to know how high I had climbed, how fast my heart beat – and aim to go higher the next day.