I don’t record every single run that I do, so I can’t tell you precisely how often I have laced up my trainers, or how far they have taken me in the last 10 years. But I track enough to know that I have run more than 1,849 times and 13,948km. That’s 8,667 miles, or about a third of the way round the world. Go me! If I wasn’t trying to eat less sugar, I would give myself a biscuit.

After all that sweating and chafing, you would think I’d have it down pat. I would have my pre-run pee, head out of the door and simply stick one foot in front of the other until I had finished whatever distance I had set out to do.

Not a bit of it. After a decade of running three or four times a week, I still stop early because I am hating it, I am more tired than I realised, or something just isn’t right. Two weeks ago, having started my morning commute with what was supposed to be a brisk 10-11km, I bailed out after 2km and got on a bus. The reason? All I can say is that I wasn’t feeling it.

Most of my runs are circuits, beginning and ending at home; I have lost count of the times I have finished them by bus or train.

I don’t feel bad about this, or about all the occasions when I leave the swimming pool early because it is too crowded or I can’t find my rhythm. As long as my overall motivation is strong, I tell myself, it is often best to stop a bad workout and save my energy for the next one.

According to many fitness freaks, however, this is Not The Way To Do Things. Whether it is leg day at the gym, or a tempo run in the park, the plan is sacred, and to stray from it is to risk all the hard work you have done to get where you are today. It is weak, it is stupid, it is a foot on a slippery slope. Next thing you know, you will be welded to the sofa, barely strong enough to pop open another tube of Pringles.

Hence the popularity of the “streak”, in which you attempt to work out every day, from here to eternity. I have gone down that rabbit hole myself, to the point where I was still doing my scheduled press-ups despite such terrible food poisoning that I couldn’t stray more than a few feet from the toilet.

And that is nothing compared with my boneheadedness 13 or 14 years ago, when I lived in the mountains and used to swim in a long, chilly lake. It was usually just two lengths, and I took them slowly, but it added up to about 3-4km – say 150 lengths of a typical leisure-centre pool. The season was short, and I usually stopped once the temperature began to drop. But this time I got in the water well into autumn. I didn’t own a wetsuit, and this was not the kind of place that had lifeguards. Even in summer, I was sometimes the only person in the water.

After 2-3km, breaststroking my way back to where I had left the car, I began to shiver. I know now that this is one of the first signs of hypothermia. The sensible thing would have been to get out of the lake and walk, but that didn’t even occur to me. It wasn’t in the plan, and perhaps the cold was getting to my brain too. So I swam on for another 40 or 50 minutes, trembling all the way. I survived (obviously), but the more I think about it, the more I see how lucky I was. If I had got into trouble, no one would have seen me, let alone saved me.

Now that I have realised that sometimes the best thing to do with a plan is to ignore it, I have been delighted to discover a lot of experts feel the same. They just don’t make a fuss about it. Take Michael Ulloa, an Edinburgh-based performance nutritionist and personal trainer. “We’re constantly told that if we can’t stick to a plan 100%, then we have somehow failed,” he says. “This couldn’t be further from the truth. It is messing people up. When we deviate from a plan, we shouldn’t overthink it. We should ask why this deviation happened and what we can do to limit the chances of it happening again. Did we try to take on more than we can chew? Are we not enjoying our current training programme? Or maybe we were just tired and we needed to give ourselves a day or two.”

“It’s easy to overanalyse and be over self-critical, but there really is no need,” he says. “Most of us are not professional athletes, we are simply everyday people doing our best – and sometimes we need to take a day when it feels too much.”

You might expect Simon Lord, a personal trainer from Oxfordshire, to be more inflexible. He has been cycling up to 160 miles a day since the start of the year, to prepare for a 24-hour charity ride from London to Amsterdam. But he is surprisingly laid-back. “Overthinking the results of one training session is common but unproductive,” he says. “Sleep, stress, diet and weather plus recent harder efforts can all play a part in performance changes, so I encourage people to remind themselves of the longer-term trends: ‘OK, today you only did three sets of 10 reps at 45kg’” – he is talking about lifting weights – “‘but look at where you were four months ago, when you could only dream of 45kg.’”

As for Amanda Katz, a New York-based personal trainer and running coach, when a client ditches the plan, she says: “I applaud them for making the right choice. They are encouraged to deviate when they’re not equipped to work out – for example, when they are feeling ill, injured, under-recovered or under-fuelled.”

Those are not the only valid reasons to abandon a workout, says Ulloa. “One that often gets overlooked is simply ‘if you are not feeling it’. I know some people might think I am making excuses or being ‘weak-minded’. But I always recommend prioritising your mental health. Some days your head just isn’t in it and no amount of kilometres run or reps of barbell squats is going to fix that. On the days I am not feeling 100%, I always go into the workout with the mindset of: ‘Give yourself permission to quit.’ Start the workout, see how you feel and then assess whether to continue. Nine times out of 10 you will. But some days you just need a break.”

In April, Ulloa was due to take part in an Ironman 70.3 – in other words, to run, cycle and swim 70.3 miles (113km). “I had trained for six months leading up to the event and I decided to pull out of the race with three weeks to go,” he says. “I have an 18-month-old son who had recently started daycare, and we had been so sick for a few weeks. This meant I had to miss a huge portion of my training schedule and I just wasn’t ready physically or mentally. I probably could have powered through, but it wouldn’t have been good for my body or my mental health. So I pulled the plug. It was frustrating, but that’s life. I prioritised my wellbeing and have zero regrets.”

I’ll be thinking about that next time I cut short a workout – or, just possibly, extend it because it is going so well. It happened the other morning, when a 14km run turned into 18km. Because that’s the other possibility that opens up when you get less fixated on the plan: sometimes you’ll hit your target – and just keep going.