One step forward, two steps a bit further forward

It's Sunday morning and I am running and I am absolutely bloody freezing.

It is the kind of morning when, if you are not a runner, you might pull on boots and a hat and walk the dog while exclaiming at the freshly-washed skies and the crispness beneath your feet, before letting yourself gratefully back into a warm house with a tingling numbness in your lips and nose.

It is the kind of morning when, if you are not a runner, you might meet a friend for a bracing stomp up a hill in the countryside, cut short with exclamations that it's actually really very cold, and followed by a gravy-splashed pub lunch and a snooze in front of the fire.

If you are not a runner, you will sample this bright, icy day with pleasure made all the more intense for knowing that you need not be out in it long.

I am a runner, so I am still out in this day. I am bowed into it like a penitent waiting to be told of absolution, bearing the lash of a bitter wind as I move my lips in silent prayer to make it stop soon.

At least I am not alone. Alongside and in front and behind me a mass of people also bob and groan -- 9,999 other people participating in this half-marathon along the ruthlessly exposed seafront.

It is becoming apparent that I have got this all wrong. I am not enjoying myself. I have completed around a third of my usual weekend run and yet I feel off kilter, off the pace, stiff and creaky. There is a recurring, uncomfortable twang at the top of my left leg.

I know the reason I have screwed this up: I am running too slowly for fear of running too fast. The last time I ran a half-marathon I got terribly excited half-way through and then almost collapsed before the finish, spent from a four-mile sprint in the middle of the race when the atmosphere and the crowds and the sense of imminent achievement soused me like cheap wine and left me staggering and gurning at the end of the party.

So this time I started at the back. Way, way at the back. Partly, I think, because I was also overawed by the companion with whom I walked to the start line: the husband of a friend who runs this race regularly; a mass of muscle and sinew who is used to knocking off one of these events in around an hour and twenty minutes and is so tough he's wearing a singlet and shorts today.

Meanwhile I am wedged behind two women who have chatted non-stop since the start (the fact that I am not out of breath either is not really a comfort at this point) and a man who is dressed as a tap. We pass only occasional supporters -- we are way out past the marina at this point and the crowds are thin here. At one point we all trot past a tall woman standing on the verge who is wearing a hat like a chimney stove and waving a giant union jack and occasionally honking an ancient brass horn to encourage us onwards. Poop, poop! she honks at us beaming, then spots the man at my side. "Come on!" she shouts in plummy tones. "Come on Mister -- what are you? A pipe? Come on Mister Pipe!" Poop, poop! Beside me I can see the man's face tighten with irritation.

At this point I check myself and look at my watch. I am running alongside a man in a tap outfit and it has taken me approximately ten minutes longer than it should have to complete six miles. I am going to have to get a move on at this rate if I am to come anywhere near the time I completed in my last race.

I lengthen my stride. I have trained for this. I should not be letting this race frighten me into submission.

As soon as I start running properly my body starts to relax and my mind tightens up: I switch on my ipod to drown out the little anxious hand-wringer inside my head who is saying -- you shouldn't be doing this, you might not make it, what if you run out of steam like last time?

Tra la la, I tell it, and turn the music up a bit louder.

At this point I spot a woman in front of me who is wearing a co-ordinated pink tracksuit, leggings and giant headphones. She has a massive rope of hair down her back and is accompanied by one man who has "COACH" emblazoned across his back, and another who keeps sprinting ahead to film her with his phone. I draw alongside and see a face with thick dark Disneyesque lashes and a hue of orange never found in nature.

I think: I can run this race faster than Katie Price, and I speed up and pass her, leaving both her and the grimacing maiden aunt in my head, flapping her hands in agitation, far behind.

By now I'm running past beach huts in primary colours and smiling families waving and cheering -- and look! there's mine, briefly, looking cold and as though they'd like that gravy-splashed lunch some time soon -- and I've got three miles to go and I'm passing people left and right and it doesn't hurt. I can feel the muscles in my legs and the strength in my arms as I push on and this is new, this is not what it felt like before -- this is a stronger, fitter me.

Two miles on and my legs feel tighter and warmer now and I am becoming more conscious of the deep gulps of air I am taking. I need to adjust my stride. I hesitate for a moment then test out a faster pace: it works.

As I run the last 800 yards and bound over the finish line I feel like yelling with joy and frustration. I complete the race in exactly the same time as my first half-marathon but the two experiences could not be more different. This has been a lesson in learning to trust my ability: the training is working. I decide to sign up for a 20-mile race in four weeks' time. I need to try again to get the pacing right. London is in 8 weeks.

Later, when I am showered and dressed, I present Grace with my medal: a chunky pewter square strung on a red ribbon which reads: "FINISHER".

She looks at it in dismay then looks up at me and asks: "Mummy, did you come last?"

"No," I tell her. "No chance."

Grace Under Pressure: going the distance as an Asperger's mum, will be published by Piatkus in October 2012