I was disconcerted for a while by that notion of Brendan being 'in the family' and I fell silent. I finished my beer and opened a second one, and then I started to feel more mellow about it all. I stood apart and looked at the family and looked at Brendan bustling around. I thought of this narrow strip of urban garden, one of dozens in the street, one of millions in London, and suddenly I was touched by the sight of Brendan going to so much effort, bustling between the barbecue – which had now been lit, quickly and efficiently by Bill – and my father and my mother. Every so often he would sidle up to Kerry and touch her or whisper something to her or give her a look, and she would light up.

He helped my mother with sorting out the marinade for the chicken and salmon pieces. Somehow he went into the house and tracked Troy down to whichever hole he'd been hiding in. He chivvied him out and persuaded him to carry plates to the table, and the different salads that Troy and my mother had made that morning. It made me think about myself and I felt a little ashamed. I wondered if I had assumed that the Cotton family existed entirely for my benefit, like some sort of museum that I could drop into whenever I felt like it. And I could always rely on other people to maintain it. My parents were there to do things for me and to blame when things went wrong. Had I thought enough about doing things for them?

By the time I was on to my third beer, I was feeling thoroughly forgiving of almost everybody in the world, and certainly everybody in this garden, though not necessarily in the most coherent way.

There was Brendan, doing five things at once, working so hard; and there was my mother bustling in and out with plates and cutlery; my father fiddling around with the barbecue to stop it tipping over; Kerry in conversation with Judy; Troy playing some game with Bill's children, Sasha and Mitch. And I noticed something odd: they all seemed to be having a good time. Brendan brought me a plate of grilled chicken and salad, and I ate it eagerly. I needed something to soak up the beer. I was so hungry that I barely noticed the very slight oddity that he had served me first. I looked over at Kerry and she sensed my looking at her in that way people do, and she turned to me and smiled. I smiled back. We were being a happy family.

I remember, when I was thirteen or fourteen, going with Bill to a house in Finsbury Park as his unpaid assistant. It was small, with poky rooms and brown furniture. We stood in the living room that had sheets over the floor, and he gave me a sledgehammer and told me to smash it through the internal wall, into the kitchen on the other side. He had to tell me twice because it seemed impossible to me that I could do this. The wall looked so solid, the room so unchangeably square and drab, and surely you couldn't just break through structures like that, so casually? But he nodded and stood back, so I heaved the hammer, which was almost too heavy for me to lift, behind my left shoulder and swung it as hard as I could into the centre of the wall, spinning with the weight of it, wrenching my arm. Plaster crumbled on to the floor and a crack appeared. I swung again and a hole opened in the surface, jagged and the size of my fist. Again and the hole widened. I could see the centre of the kitchen, a draining board and sink with dripping taps, and beyond that a fractured piece of the small garden, with a bay tree at the end of it. And I felt all at once tremendously excited – to be opening things up like this, new vistas with each swing of the hammer, light suddenly flooding into the dreary room. I think it was what first made me think that I'd like to do what Bill did, though when years later I tried to say that to him he patted me on the shoulder and said: 'We're just painters and decorators, Miranda.'

Every so often at work I still had that feeling of euphoria – like a bubble of air in my chest, like a wind blowing through me. I got it, for instance, with the roof garden in Clapham, which somehow took the lid off the whole house. And when we once uncovered a fireplace that was so vast you could stand inside it and look straight up to see a penny-sized circle of sky at its top. Knocking walls down always fills me with fresh energy. And every so often I have the same elation in my personal life too. It goes along with transition and change, spring, falling in love, travelling to a new country, even that feeling of newness that comes after an illness.

After that lunch, I came home and I made two resolutions: I was going to clean up my flat and I was going to start running. That was all. But I wrote them both down on the back of an envelope, as if I would forget otherwise, and then I underlined each of them twice. I sat back in the chair. I'd drunk three cans of beer and eaten two pieces of marinated chicken, a slab of charred salmon, three slices of garlic bread and a bowl of ice cream. If I were being really virtuous, I could go for a run right now, before it got too dark. Or maybe it wasn't healthy to run on a full stomach. And anyway, I didn't want to jog along the high street in my grey tracksuit trousers that had lost the elastic at the waist.

So I thought I'd start with the flat. I changed into some baggy trousers and a sleeveless T-shirt and put on some music. I rather like tidying my flat, which is on the first floor and very small – just my bedroom, the living room, with a table up against one wall, a galley kitchen with windows overlooking a patchwork of narrow gardens, and the bathroom. Clean surfaces, dishes all put away, a vacuumed carpet, washed floor, neat piles of paper on my desk, laundry in the basket, clothes back in the wardrobe, the gleaming bath, the pens in a mug on the mantelpiece, the smell of bleach, polish, lavatory cleaner, soap. My bare feet were gritty and my arms and forehead slick with sweat by the time I'd finished, and it was late. Afternoon had become dark evening and, now that I'd stopped racing around, I could feel the air had the slicing chill of a cloudless October night.

Some of my friends don't like living alone. It's what they're doing until they no longer have to. But I do. I like the feeling I get when I close the door behind me and go upstairs and everything's quiet and waiting. I don't need anybody's permission to lie in a bath for two hours or go to bed at half past eight or listen to music late into the night, or pour myself a glass of wine and watch a trashy quiz show. I even like eating alone, though I'm not like Troy. I have a very limited and conservative repertoire. Sometimes I eat the same thing several nights a week – for a bit it was scrambled eggs on very buttery brown toast. Then it was Greek salad, which I've perfected: not just tomatoes and cucumber and feta cheese, but avocado, fennel and sun-dried tomatoes as well. And there were a few weeks when I would add a tin of octopus chunks to a bowl of tinned chickpeas. I went off that one quite quickly. When friends come round I either cook chicken breasts with garlic, rosemary and olive oil – you just have to put it in the oven and wait for half an hour – or we get a takeaway. Usually it's a takeaway.

Maybe one of the reasons that Brendan had got on my nerves when we were going out was that he had so quickly made himself at home in my flat. As if it were his home too. But I told myself not to think of Brendan any more. Things were going to be different now.

At a shop called Run Run Run in Camden High Street, I bought a rather lovely silky blue singlet, a pair of white shorts, black suede shoes and a book called Run for Your Life, It was written by a man called Jan who appeared on the back of the book wearing a headband, like a member of Duran Duran. Then I went to the off-licence and bought a bottle of white wine, cold from the fridge. Nothing that was so transparent could possibly contain a significant number of calories. And I bought a packet of expensive crisps that the packet said had been fried in an especially healthy kind of sunflower oil. I fastened the chain on the inside of my door and lay in the bath with a bowl of the crisps and a glass of the wine and read my running book. It was very comforting. The first chapter seemed to be aimed at people who were even less fit than I was. It suggested starting your running schedule with a brisk walk for ten minutes and then running very gently for a hundred yards, followed by another ten-minute walk. It said that the training runner should never get seriously out of breath. At the first sign of any kind of discomfort, just stop. The fatal thing was just to set off and go for a run. 'Better to start too slowly and build up,' said a piece of text in italics, 'than start too quickly and give up.' That sounded fine to me. I flicked through a few pages. It looked like I could skip a few stages and still avoid breaking into a sweat.