Fatigue is therefore not a physical phenomenon, but an emotion that can be controlled, argues Laura Walsh, former Hong Kong triathlete and mental health sports counsellor. "However bad you feel, it's your brain playing a trick on you, trying to make sure you don't damage yourself." So, to keep going or run faster, you need to learn to activate a larger mass of muscles than your brain is allowing.

Locating that switch and developing "mental endurance" is the key to finding new limits and conquering your marathon goals, says Walsh.

"Mental endurance is the ability to 'carry on' when you feel your body is telling you to stop. It's about understanding that the body is in fact able to continue, instead of allowing the physical signals to be interpreted as the necessity to quit."

While there are, of course, physical thresholds which our bodies cannot exceed - high core temperatures, dehydration, lack of energy - that point is in fact beyond where your brain is telling you it lies.

Notwithstanding all of the above, mental strength is no substitute for training, however. "Mental endurance only comes after the physical advantages from training, such as weight loss, increased lung capacity, stronger muscles and so on have been gained," Walsh says. Once you have acquired the necessary fitness, you can work on this next stage of performance.

Reprogramming the brain's limiting messages begins with pinpointing your motivation to run. "You need to understand and know the big 'why' behind why you are doing the race in the first place, otherwise it's too easy not to fight the temptation to quit," she says.

The next priority is to visualise the race and imagine yourself finishing. "Before the gun is even close to going off, our brains have decided the outcome," says Walsh. "Visualisation is extremely powerful in keeping the mind focused on the present," she says, suggesting you focus on positive images such strength, endurance, efficiency and speed while you're running.

Trainer Clinton Mackevicius is fond of visualisation techniques while racing. "I visualise how I want to be feeling at certain parts of the race," he says. "In order to relax while running fast, I also think back to previous sessions where I can sync my running gait into a pattern that allows me to run easier without losing pace."

Keeping positive is essential, continues Walsh. She suggests writing down positive affirmations and rehearsing them before the race. "For example, if you have difficulties running up hills, instead of cursing and thinking how awful the hills feel, write down how you will cope with the hill and what you will say to yourself, like 'shorten your steps, lean into the hill'," she says. "Same scenario, just different thoughts and, most likely, results."

Mackevicius, too, keeps a positive mantra in mind when he runs. "I never lose sight of the finish line when I run I and never let doubt enter my mind," he says.

Rather than telling yourself to not think about running such a long way, Walsh says staying focused will help you to work through the mental obstacles you will encounter. "The brain doesn't understand a negative, so by focusing on not doing something, you are in fact doing it."

Devising a race plan can alleviate anxiety on race day, while keeping your mind engaged, says Mackevicius. "You should always go in with a race plan A, and also a plan B if things don't go accordingly," he says. By actively telling the brain what to do, you will remain in the present and have greater control over your emotions.

Walsh reminds runners that pain is a subjective experience that can similarly be controlled. "To begin with, change the word you use to describe this sensation. Pain has such negative connotations that creates anxiety and heightens the feelings you are experiencing," she says. Just as we can train our mind to work beyond the urge to quit, we can retrain how it interprets pain. "Instead of thinking of pain as being awful, understand the sensations you are feeling are simply your body's physiology of hard effort."

Above all, have an unwavering belief in your potential. "The truly great runners don't always have the highest VO2 max, or the greatest cardiac output (we wouldn't need to have competitions if this was the case; it could all be decided by numbers in an exercise physiology laboratory)," says Walsh. "The true greats have learned to look at the discomfort of the event as a challenge to be overcome, and thus have that fantastic ability to never, ever give up."

Rachel's training diary

I completed my first long road run this week and it wasn't as daunting as I had anticipated. I stuck to my instructions on keeping up my nutrition during the run and I never really felt tired. I did sneak in a few water stops and checked my phone on the way (I still find running for that long a little tedious), so my pace was a little bit off, but I feel like I'm on track. Next week's big run will be the true test.