26.2 miles

When I think about Sunday, April 22, the first thing I think of is the noise: the boom and clamour of the crowd all around, as if I were caught in the centre of a rolling wave that hurled me along and along; the whoosh and thump of my heart behind my eyes and in my ears; the steady one-two-three-four of my breaths, matching my stride; and behind it all a whisper, an insistent murmur behind the din and the hot roaring pain that came later, a voice that said: Grace, my Grace, my girl.

The night before the race I travelled to my sister-in-law's flat, a tiny fairytale garret at the top of an old Victorian house, that looked out over south London and the start line, fifteen minutes' journey away. All that day I had been distracted and quick-tempered, pretending to be a participant in Saturday family time while ticking off the hours in my head before it was time to leave. When I could finally go and pack it was a relief, even though, checking and folding my clothes, I felt more as though I was preparing for the executioner's block than the winner's podium. It took an age to pin my running number onto my vest -- my hands were shaking so badly that I fumbled the safety pins and pricked my fingers repeatedly. When I came downstairs to say goodbye to everyone the children stood awkwardly in the doorway, half-turned to run away and play again, half-aware that something was expected from them in this moment. A good luck card was produced. I hugged and kissed them all and bent to pick up my kit bag and then Grace was in my arms again. Voice muffled in my chest, she said: "Do this for me, mummy." I held her out from me to look at her face, ready to say something light-hearted and reassuring when, solemnly, she raised three fingers to her lips and held her hand out in salute to me, mimicking the gesture of love and respect used by her new heroine from the Hunger Games. It was so Grace: dramatic and funny and sweet, and heart-breaking.

That night I slept badly, tossing and turning for hours on the edge of nervous dreams that threatened to throw me into wakefulness. I rose at six, ate porridge and brushed my hair looking in the mirror at my terrified face and listening to the radio, which seemed to be playing in another country -- who were these people who could laugh and joke and comment on the lovely weather, preparing for a relaxed day of newspapers and walks and roast dinner? Outside the sky was clear and I walked down the hill towards the train station alone and in silence, avoiding Saturday-night piles of sick and litter. At a bus stop a woman lit up a cigarette. Then I turned the corner and there were loose knots of people with red race-day kit bags, waiting for the next train. Among them was a friend and fellow runner for the National Autistic Society. The tightness in my stomach loosened.

I don't remember very much about the start, except the mass of people around me and in front of me and behind me and the calming voice of my friend, who had done it before, and kept me distracted with stories. Helicopters buzzed overhead and cameras turned on us and a voice on a tannoy urged us to repeated cheers and whoops as we waited awkwardly, nervously, for the countdown to the start. Then there was a walk, a shuffle, people bouncing on their toes, discarding jumpers and waving goodbye to friends and somehow I'd got to the start line, a huge arch bright red against the blue sky and the pliant mesh of the timing board spongy beneath my shoes, activating the chip tied into my laces, and I was running.

Almost immediately the crowd was there, still modest at this point, strung out like beads along the barrier, sending us good luck and smiles as we set off. We passed an elegant Georgian home in the garden of which two young people on brass instruments puffed out a slightly melancholy version of the Rocky theme tune. I found myself pacing behind a purple Teletubby and in the time it took to puzzle his name -- what was it again, was he Po or Tinkywinky? -- I realised the first mile was done and I was running the marathon and bobbing along in the centre of a crowd all streaming forward in glorious colour and purpose. For the next few miles we passed several churches which had opened their doors to bless us and cheer us. In front of one, a vicar swung holy water, sending droplets arcing out over the shifting mass, and we raised our hands back, a communal thank you of hundreds. Then there was a choir and a band, and then another band, and then a string of pubs all open, with people dancing on wooden tables outside and waving at us.

So the miles passed. We came to our first incline and as one we all bent, and suffered just a very little bit, and turned to smile with relief at each other as we came over the brow. I found my pace with ease, checking my watch now and again to make sure I wasn't going too fast, or too slow, and finding every time I checked that my body was now automatically doing what I had trained it to do all those months and was carrying me forward with ease. A bit of me floated away and just watched the carnival around me as I progressed. Then the route turned right, and we all turned right and suddenly in front of us was Tower Bridge and I'd run twelve and a half miles and was passing under the grey turrets and the crowd was going crazy. Down along the highway the crowds were deeper and deeper -- three or four back from the barriers and screaming and shouting. It was a huge effort not to speed up, for I was loving every moment, grinning like a loony, knowing that my family were going to appear at any moment at mile fourteen, where the National Autistic Society had organised a cheering point. I ran with my neck craned, seeking out the purple and white and red balloons and banners and feeling goose bumps running up along the back of my neck in anticipation. And then there they were -- only on the other side of the barrier where I couldn't touch them -- so I yelled and jumped up and down, pumping my arms in victory and blowing kisses and the roar that went up was for me, for me and for Grace and for us all and as I ran away from them all I was overcome and saw the route ahead of me blurred for a while.

When I came to again it was mile fifteen and something wasn't right. Before I had time to realise what was bothering me I felt a hand on my shoulder, the runner behind me directing me to a voice in the crowd to my left. It was my friend and former running partner Karen, who had started with me nearly a year ago, puffing and blowing and swapping stupid jokes with me on those first training runs as we contemplated our first half-marathon. I shrieked with joy and ran to her and she grabbed me. Both of us wild-eyed and teary, we exchanged kisses and loving words -- none of which I can remember now -- and then I was running again, careering forward in a state of such massive emotion that I'm amazed, thinking of it now, that I didn't spontaneously combust on the spot.

By sixteen and a half miles I'd figured out what wasn't right. The pain in my back had bloomed again, despite those weeks of enforced rest and expert osteopath attention, and the first corresponding shivers of pain were sending feelers down my right leg. A swooping downward lurch of panic hit me. I tried to shake it off and keep going but the pain was building very quickly and with it, my distress.

So I did what you do when the going gets tough and the tough need to get going: I went for a wee and a think, veering off the course to where a line of portaloos stood and barricading myself inside one to shut in my panic. There, in the dim blue light and the animal smell of other people's fear, I chewed painkillers and took shaky breaths and thought: how do I do this, how do I get going again. The murmur told me: Grace.

So I came out and I started running again, only I couldn't. I told myself I would walk half a mile and then try again. I counted down to seventeen and a half miles and started running again. But the pain built up again, so I walked and hobbled, pushing out thoughts of failure and ashamedly hoping that no-one would see me walking, and then I forced myself to run again. By now I'd passed the eighteen-mile mark and there again was Karen, yelling at me from the barricades. Sagging with relief and self-pity I went to her and fastened myself around her neck and told her, choked, how much it hurt. She hugged me back tightly and told me how well I was doing. Around us the crowd looked at our embrace and looked at my face. Arms came out and patted me and told me I could do it. A blur of faces pushed into mine and said, come on, come on. Karen released me back into the flow of runners and I found my pace and ran again.

The next three miles were misery. At this point the route had taken us into the heart of Canary Wharf, all hard-faced glittering windows and no progress: we wound around and around, marking time and miles until we could be released back towards the centre of town and the final stretch. But not yet, and not yet, and not yet. We passed restaurants and wine bars and offices -- including my own place of work, an incentive to run straighter and a bit faster, no matter how much it hurt -- and here the crowds shouted for Toby and Robert, and yelled "Come on old man!" and rugby shouts of "Whoooooarrrrrgh!" for confident young specimens who strode out and overtook all around me.

Then, thank God, we were released, leaving the glistening maze behind us and heading back out towards my family and friends and the NAS crowd of supporters who I knew were all waiting at mile 21, scanning the crowd anxiously, their gazes turned so far out into the mass as they searched for me that they jumped when I came up the inside lane and whooped in their faces. Ecstatic, I did a little dance and they laughed, and grabbed me. My parents and sisters, tears in their eyes, clutched me. My husband leaned forward for a kiss and Betty and the boys -- hot and sticky -- smiled and put their hands out to me. And there was Grace, smiling and kissing me and asking me: "Why are you crying?"

At that point I thought I'd done it. Leaving everyone I loved behind me with a wave and a promise to see them at the finish, I thought I'd cracked it. I ran on, back along the highway -- the roar building to an astonishing pitch -- and into Blackfriars tunnel and an incline to the Victoria Embankment. As I bent into it the pain in my back came over me in a huge wave. For a panic-stricken moment I thought that I was going to be sick or black out. Breathing, concentrating, I emerged from the tunnel sandwiched in the middle of the staggering pack, to an immense crowd screaming encouragement. A sign told me I had only two and a half miles to go. Around me I could see runners smiling through their exhaustion and managing a final spurt. But all I could manage was a hobble, a limping half-walk, and then a walk -- biting my lips not to cry in front of all these people. My watch showed that I still had twenty minutes before five hours had passed. I walked as quickly as I could bear to, hoping to ease out the pain enough to pick up a run again, and started a distracted, frayed conversation with a similarly wrung-out runner alongside me.

At the corner of Big Ben I turned and saw, if it were possible, an even bigger crowd. I started running again, unable to bear the humiliation of walking in front of so many, despite being almost cross-eyed in pain. The route turned into St James Park and along Birdcage Walk, and while my mind was yelling at me to stop, it was also registering dimly that this was nearly the end. A sign said 800 metres and I nearly threw myself down on the floor at the thought of how far I still had to go. But then I turned and there was Buckingham Palace and the Mall and people were ten deep at the barricades, hanging off lampposts and thronged, waving, in the fountains and there was no way I could stop in front of them all, so I plodded on, weaving and bent, and there in front of me was another red arch like the one I had gone through nearly five hours ago. The clock on top of it said four hours and fifty-four minutes. I headed for it and was aware that I was smiling, that we were all smiling, and that it was nearly the end.

I crossed the finish line and raised my arms intending to make a victory gesture but found instead as I slowed to a final stop that I was clutching my head and weeping. Wiping my face with sticky hands, I followed lines of shattered runners submitting to the attentions and directions of race officials. A young man removed my time chip from my shoes while a woman strung a medal around my neck. In a daze, I collected my kit bag, slung it over my shoulder and started out into the crowds again to find my daughter.

Grace Under Pressure: Going The Distance as an Asperger's Mum is published by Piatkus in October 2012