A s a 16-stone, heavy-drinking, asthmatic, smoking slob, I decided to throw myself into a gruelling training regime. The goal was to make the cut for the marathon at the 2008 Olympic Games in Beijing - and make a documentary of my journey. Why? Credibility. Of course ambition, the joy of running and personal challenge all played their part. But when the Road to Beijing project culminates at the finish line of this year's Flora London Marathon, I will be able to look back and say I gave it everything.
I always hated running as a child. I have particularly bad memories of the obligatory cross-country sessions at secondary school. Wheezing and fatigued, I'd lumber around the course, often turning the final corner to find that the finish line had been packed away and everyone had gone - a demoralising and humiliating experience that left me with little motivation to ever run again.
By 16 years old, all I did was play video games and eat junk food. As a result, my weight had ballooned to 17 stone, and my doctor explained that I was facing a lifetime of obesity-related illness. The news hit me hard, and I resolved to turn my life around. Under the cover of darkness, I started to run a three-mile circuit around the school three evenings a week. Soon the weight started to drop off, and I gained confidence - not to mention extra energy. Crucially, I started to enjoy running.
Aside from a brief blip, as you might expect of someone in their first boozy year at university, I kept up my running. In 2001, I entered the Flora London Marathon. No one thought I would be able to get all the way round. Crossing that finish line for the first time still rates highly as one of my best experiences ever. OK, 4:21 hardly made me Gebrselassie, but it was a start. Two marathons down the line and I had cut my PB down to 3:35. But I genuinely thought that I'd peaked - that I couldn't possibly go any faster. Exasperated, I contemplated hanging up my running shoes and moving on to a different challenge.
I didn't have to wait long. While sitting my finals, a friend suggested that we enter Marathon des Sables - the infamous seven-day, 150-mile footrace across the Sahara. I thought he was crazy, but the next day we signed up, and six months later we were baking under the Saharan sun on the start line. I nearly died of dehydration twice on the way to the finish line. But the experience taught me an important lesson: your body can give more than you think it can.
In October 2005, my running regime really hit the brakes. I had developed a Pilonidal sinus - a group of ingrown hairs at the base of the spine. It may not sound serious, but six weeks on from the operation to remove them, I was still recuperating in bed. Fortunately, it gave me the time I needed to cook up my new challenge - one that would push me to my very limits.
It dawned on me that with the exception of Paula Radcliffe and Jon Brown, I'd barely heard of any British marathon runners. Was it just the case that there weren't any? And, if so, why? I did my research and discovered there were a few very good British marathon runners, but none were even close to being in the same league as the Kenyan or Ethiopian runners. In 1985, 105 British men had run under 2:20 for the London Marathon, but only a handful had managed the same feat in 2007. As a nation, we appeared to be getting worse at the 26.2-miler.
As a filmmaker this subject captivated me. I started to wonder how far I could I progress as a marathon runner if I threw myself into training. I was convinced it would make a compelling documentary. I wanted to give the audience an idea of not only of how hard marathon training can be, but how far you can progress with a little talent and a lot of determination - and maybe even qualify for Beijing. Pie in the sky? Perhaps. But I took the attitude that anything was possible until proved otherwise and I immediately started training and filming.