Andy Jackson, 20, a student from New Malden in Surrey has leukaemia and underwent a transplant within days of completing last year's marathon.

In the summer of 2003 I was working part time for an engineering company in London and remember feeling exhausted when I got back home every night. I was also getting headaches and dizzy spells but put everything down to my burning the candle at both ends. I was also still coming to terms with my mum's diagnosis with terminal liver cancer.

I was due to start a history and politics degree at Exeter university that October and so, on my mum's insistence, I went for a general check-up with my GP. Within hours of that appointment my life turned upside down and I was being rushed by ambulance to see cancer specialists at Hammersmith hospital. Consultants confirmed later that day that I had late-stage chronic myeloid leukaemia and must stay in hospital for urgent treatment.

I had to postpone my university place for a year. It turned out that my leukaemia was so advanced I had probably had it for at least 12 months.

I was put on a drug called hydroxyurea - a mild form of chemotherapy - and had a bone marrow sample taken and then waited for a donor. During that time, for my mum's sake and my own sanity, I felt I had to do something positive. So while I was still on treatment I entered the 2004 London Marathon and began training for it. Over Christmas 2003 I was told a donor had been traced but I pleaded with the hospital to allow me to postpone the transplant until after the race. Running became my focus and took my mind off the illness. I completed the marathon in under four hours and raised more than £15,000 for charity.

After that I had a couple of weeks to recover before being admitted to hospital for three months where my cancerous blood cells were blasted away and replaced by healthy bone marrow. I was eventually let out of hospital on June 2 last year, but my mum died less than a week later. She is one of the main reasons I am running again this year.

Jodie Hyde, 25, a student and single mother from Folkestone in Kent, is running her first marathon. Five years ago she was a recovering heroin addict on a methadone programme

"After I left school I got in with the wrong crowd in my home town of Folkestone. Then I met a boyfriend who seemed caring and we moved in together. I didn't know it at the time but he and his brother were both on heroin and they introduced me to the drug, first by putting it in a joint without telling me. Over the next few months I got hooked on it myself. I smoked it from foil for while, but pretty quickly progressed to injecting.

My mum begged me to go to the doctors with her and it was a turning point. It was tough coming off heroin and took almost a year before I was off the methadone, but I gradually got a grip on my life again. I got a job and began to make new friends. Now I am studying for accountancy qualifications at a local college.

Last year I watched the London marathon and decided to do it, even though I'd never run before. I went for my first run ever last May and it was short, but pretty tough. I didn't have any idea how to train for a marathon, so I went to a local running club for advice. When I can, I join them for training on Tuesday or Thursday evenings and try to do a longer run with them on Sundays. But I'm just as happy running on my own.

I'm now a running addict. It is the best form of stress relief I've ever found. When things get on top of me, I put on my trainers and get half an hour of fresh air, even if it's snowing or raining outside. When I'm out there I'm on top of the world. I have built up gradually by increasing my mileage and I now run for an hour at least a couple of times a week. Last weekend I did a 20-mile run which will be the longest I do before the big day. I'm taking the marathon seriously and aiming for under four hours. My life has turned around and I want to prove to myself that I can achieve what I want to."

Emma Leigh, 26, is a radio journalist from Sevenoaks in Kent. She was told she may never walk again after breaking her back in a riding accident

At 16 I was a mad keen rider when I had an accident going over a jump. It looked nothing spectacular - the horse just jumped and fell badly - but I was instantly numb from the waist down. After endless tests they discovered that I had fractured the L4 vertebrae at the bottom of my spine and suffered slipped discs. The consultants ordered me to take complete bed rest in hospital for a month. They said they didn't know how long it would take for me to recover, but that I had to lie completely flat.

After a month I began to get some sensation back, but even then I couldn't stand up. With the back pain and muscle wastage, my legs just buckled when I shuffled around with a Zimmer frame. It took six months for me to be able to walk even a few steps to the end of my drive and back.

It wasn't until 2003 that I was really able to contemplate doing anything sporty. I entered a 10km fun run and, although I had postural problems and pain down one side of my body, I completed it. That got me thinking about the marathon. I wanted to do something that I could view as a payback.

Training hasn't been easy - I am in a lot of pain and always at the physiotherapists. But my inspiration has been Christopher Reeve who had his accident at the same time as me and should never have lived, let alone achieved half of what he did. When I'm having a slob day, I think about how lucky I am to be able to run at all.

It is the first time I have attempted anything as physically challenging as running 26 miles. And I'm that adamant it will be the last."

Irene Clare from Epsom is 79 and the oldest woman in this year's race. She took up running when she was 58

"I had never been very sporty, but when I was 58 I got the urge to enter a local five-mile fun run in Epsom. I had no idea what running was about, and just bought myself a pair of trainers and went out jogging a few times to get fit for the race. My family thought I was mad. But since then I have been hooked. I joined a running club, called the Epsom Allsorts, and still run with them regularly. I'm getting slower, but I manage to get out most days and to run 10 to 11 miles with some of the girls from the club on Sundays. What I do find as I get older is that my legs feel like lead for the first three miles. I have to talk out loud to my body to persuade it to get moving. Once I'm into it, though, I can keep going for a fair distance. I love the way I feel when I am out there - there is nothing like it. I do other things including yoga and swimming and I'm a member of a gym, but running is my passion. Even when I am not training for marathons, I do shorter races to keep my competitive edge.

This month, however, I am doing a 20-mile race which will give me an idea of how quickly or slowly I can expect to finish London. Usually I finish in the top three in the over-70 group and complete it in around six hours, although my fastest time is five hours and five minutes.

This year's race will be my 14th London and my 21st marathon; I once ran two marathons in a month. In the past 10 years I have run in races everywhere from Athens to Boston, Berlin, Rotterdam and New York. But there are plenty more to do. I'd like to have a run in the Dublin marathon. My family sometimes think I should start slowing down and taking life easy. But I've got far too much energy to burn. And I feel as fit as a fiddle. If I didn't run I'd probably age dramatically overnight."

· Andy is running for the Leukaemia Research Fund (www.lrf.org.uk)

· Jodie is running for Action on Addiction and can be sponsored at www.justgiving.com/jodiehyde

· Emma is running for Spinal Research and can be sponsored at www.justgiving.com/emmaleigh