David Walliams’s murky marathon

The majority of my training has been done at a 25m pool in central London and it’s all about putting in the hours. There’s no cheating, no way of short-circuiting it; what’s required is length after length, stroke after stroke. No matter how tempting it might be to miss a session or skip a few lengths, I know from experience that, come the challenge, I’d regret it in a big way, so there’s nothing for it but to plough on and on.

I’m more nervous about doing this than I remember being about swimming the Channel. The Thames’s unpredictable currents, strong tides and river traffic make it treacherous, especially when it becomes tidal past Teddington Lock – many people, including strong swimmers, have drowned. I know I’ve got to be in the best shape possible, not just to have a chance of completing the distance, but of coping with the powerful pull of the water itself.

Although I’ve always loved swimming and found it to be both peaceful and therapeutic in many ways, I’d be fibbing if I said that the training doesn’t get a bit monotonous at times, which is why I jump at the chance of changing venue whenever I can.

An opportunity to do just that came my way in July when I entered the Great Marlow Swim at Higginson Park, Buckinghamshire, a 3km event that sees experienced triathletes, hard-core Iron Men and open-water specialists pounding down the river in a mass of flailing arms and legs.

The thought of little old me being left for dead by these impressive specimens wasn’t something I was relishing and I had visions of trailing in last as the other competitors were packing their cars to head home. But I held my own and came in a very respectable 13th out of 59 in 47 minutes and 29 seconds.

Once I realised I wasn’t going to embarrass myself I started to enjoy it, and it actually ended up giving me a lot of confidence. It was only a fraction of the distance I’ll need to swim in order to complete the Thames challenge, but despite my nerves I was a happy boy – and I got a medal at the end, which is always nice, isn’t it?

After another few weeks of pounding away at my local pool I got a call to say I’d been invited to train at the brand-new Olympic Aquatic Centre in east London. It had only recently been opened in a blaze of publicity and I couldn’t resist the chance to swim there.

Driving through the Olympic Park, which was a hive of activity, I arrived at the ultra-sleek building, on a site which just three years ago was a marshy waste- land. The dazzling interior opens up as you reach the water’s edge to reveal a vast bank of 17,500 seats rising up into the heavens.

The 50m competition pool, which is a thing of beauty, is a consistent 3m deep from end to end and is fitted with a hi-tech system that prevents the waves you create from rebounding back on you. The 2,500m I completed flew by as I glided through the water, heated to the optimum temperature for speed.

As well as admiring the stunning surroundings I couldn’t help thinking that as I was one of the first people to try the pool, hardly anyone would have done a wee in the water – in stark contrast to the Thames, which is often flooded with raw sewage when heavy rains hit. Needless to say, I’m hoping for sunshine.

Towards the end of the session I was joined by British swimming legend and BBC commentator Karen Pickering, who gave me a few tips. Having swum in the Thames herself, she told me that she thought river water tasted better than pool water, which made me wonder what kind of places she’d been forced to compete in over the years!

Before I left I couldn’t help but take a look from the very top diving board, where in under 12 months’ time Tom Daly will perch as an expectant nation looks on – rather him than me.

The day after my Olympic adventure, I visited a Sport Relief project just a stone’s throw from the Thames in Battersea, south London, to see how the money I’m hoping to raise will be used by Sport Relief here in the UK.

The residential centre offers a whole host of activities for older people and I shared a cuppa with three delightful residents, Violet, Doris and Alan, who have all benefited from sessions that encourage them to reminisce.

Tales of eels and boating trips to Margate came thick and fast and I promised to look out for the hat that blew off Doris’s head on an outing 75 years ago. Alan, a 70-year-old recovering stroke victim and former Merchant Navy cook, remembered as a child that being caught swimming in the polluted Thames meant an immediate trip to hospital to have your stomach pumped. It’s cleaner now, of course, but I’ve been advised to keep my water intake as low as possible.

So now, all that remains to do is put the finishing touches to my training and come up with a way of taking my mind off the pain. During the Channel swim I listed every one of the Pet Shop Boys’ singles in order of release, over and over again in my head – maybe this time keeping an eye out for Doris’s boater might do the trick.

Hopefully, once word gets round, people might be good enough to come out and cheer me on as I swim past – with that support and the constant updates on how much is being raised, I should be able to keep focused and complete what will undoubtedly be the hardest challenge of my life.

Follow David’s progress as he swims down the Thames and make donations at: www.sportrelief.com/walliams