David Flatman is preparing to suffer this Sunday © Getty Images

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On Sunday, 20-stone ex-England prop David Flatman will take on the Bath half-marathon. He agreed to run the 13.1 mile course for the Down's Syndrome Association, a charity close to his heart as his brother suffers from Down's Syndrome, and while the idea was at first appealing, the brutal reality of what he is about to embark on is slowly hitting home.

"In terms of peaks, there hasn't been one yet," Flatman told ESPN. "The peak will be watching Top Gear in Sunday night, hopefully not dead. The troughs have been more numerous like when I went for my first jog after agreeing to do the half marathon. I tried to push myself a little bit as I ran in deep mud through the floods. I felt like Rocky Balboa training in Russia.

"Actually, I didn't run very far, found it exhausting and got ill. I stayed ill for three weeks and this Monday last gone I woke up with all my antibiotics finished and I felt okay and ready to go. I thought 'I can now train for the half marathon' - which was in six days. I have been running twice this week and all of my runs added up don't equal 13 miles. That gives you some idea of the challenge I'm up against."

Last summer, alongside Danny Grewcock, he cycled from John O'Groats to Lands End for the Bath Rugby Foundation and they raised just short of £11,000, but he felt the effects for a good couple of months afterwards. Flatman has only lost two pounds while training and there are more pressing concerns passing through his head such as toilet stops.

"Ultimately I'm not comfortable cycling because I'm too heavy and I don't like endurance sport much. I'm more comfortable cycling than running though as it doesn't hammer my joints. This will be tough on my ankles, knees, hips, back and neck. What I've been told to plan for is extreme pain early on which gets worse and a few toilet stops. If you're a heavy eater and you're not used to bouncing up and down it does weird things to you so you need to plan the toilet crawl. I don't want to do a Paula Radcliffe and have a bowel-related incident on the Lower Bristol Road."

The support for Flatman's challenge has been huge. He originally said he would run the half-marathon if he raised £3000, his current total has more than exceed that target.

"It'll be very nice to give them a cheque for the best part of £7000 on Sunday night. Ultimately, if I die they still get the money. There are no refunds in case of death so for the Association, it'd be great PR for them if I died in their name and the money could cover the funeral.

David Flatman with his brother Christian © David Flatman

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"I've had a completely unexpected level of support. We've raised nearly £6500 and I only signed up four weeks ago so it's been amazing. The support has been unreal with some really generous people with a lot remaining anonymous. But Matt Giteau and Drew Mitchell have chipped in - I met those guys once and we had a great day at Twickenham over a few pints and a laugh. And there's no reason for them to sponsor me a lot of money, but they have. I'm sure there's something sadistic in that as they enjoy my suffering, agony and lack of preparation. Lots and lots of people have been generous and it's been amazing.

"I got a message off a chap on Twitter which said 'look, we've never met but I love what you're doing and the cause and whatever you reach, I'll round it up to the next thousand quid'. There's no reason for him to do that but people seem to really relate to the Down's Syndrome Association. My brother has Downs so people know it's real and that makes a big difference."

To donate to David Flatman's cause, click here

Update: On Sunday, Flatman tweeted to confirm he had successfully completed the half marathon.

Didn't win the @bathhalf which is disappointing. Did run every mile, though some elderly/smug athletes seemed to be walking past me. Ruined.

— David Flatman (@davidflatman) March 2, 2014