I have run 20 marathons in 12 years. You’d think I’d know what I am doing by now. Yet I forget two things every single time. The first is just how far 26.2 miles actually is. Ridiculously, mind-bogglingly, painfully far. So much further than you think. It surprises me every time. Particularly if I am chasing a PB. The other thing I forget about without fail are the Marathon Demons. They pounce on me when I’m least expecting them. This is how it tends to go inside my head…

Thirteen, fourteen, fifteen miles in – I’m still feeling good, pacing well, perhaps a bit too fast? Pull it back, rein it in a little. The training is done. I’m on track. A PB on the cards? Think of this as the victory lap. Feed off the crowd, the noise, the faces willing me on. Listen to the sound of thousands of feet hitting the tarmac, almost in synch. Metronomic. Heart rate is good. I can keep going at this pace. This is good. BAM! I cannot do this. I’m too old. Why do I keep putting myself through this pain? What am I trying to prove? I want to pull up, push through the crowd, put my head in my hands and cry. My watch is beeping. Oh look! Another mile ticked off. I’m still on pace. Actually, I’m going faster than I thought. Hey, this is okay! Keep on it. I can still do this. My legs actually don’t feel that tired. Ten more miles to go. I can visualise that. I can do this! Think of the finish line, the medal. BAM! Another hour of this? I am so tired. I want to stop. My heart rate is rising. I can’t hold this pace. A water station. Don’t walk it. Do not walk it. Do not stop. I am walking. Why am I walking? Twenty seconds walking. Running again but those were still precious seconds I’ve just lost on the clock.

And so it goes on and on and on in my head. The mental battle I fight every time is almost as exhausting as the physical one I pound out on my legs.

London Marathon is next. So I’m turning to the experts to help sort out my Marathon Demons out. First up, I try Rose Harvey, one of the UK’s fastest female marathoners. How does she keep her mind calm while trying to finish in under 2 hours 23 minutes? ‘I have a mental plan for my race,’ she says. ‘The first half is just about enjoying it. Soak up the atmosphere, look around, engage with the crowd. It probably won’t feel that hard. From mile 13 to 20 is when I focus on the people in front, on my arms, my breathing, just to keep it as easy as I can. Mile 20 is when the race starts, when the negative thoughts may creep in. So I have an action plan. If I hit a bad patch, I force myself to smile. I count to five and make myself smile for those whole five seconds. And I remind myself just how lucky I am to be doing this – in an elite race, in this place I never thought I could get to. And normally, by then, I have thought myself out of the hole.

Next, I try top coach Jo Wilkinson, an ex-GB international herself. She is a little more brutal. ‘It’s a mix of mental bribery, deceit, reward and kindness in rotation,’ she says. ‘Remember you are never ever going to feel as bad at the end as you do in this moment now. So just keep going because you don’t want to feel that horrible disappointment that you gave up. One step further is one step less – that’s often my mantra. Ultimately, you know just how amazing you are going to feel when you finish, so keep pushing. Oh, and I do swear a bit as well. “Just keep ****ing going,” is what I shout in my head. But don’t print that...”