The 10-marathon river stretch

The group of 36 "extreme runners" are taking part in Britain's hardest "ultramarathon" - the 250-mile Thames Ring.

It is said to be harder than the notorious Marathon des Sables endurance race across the Sahara and is five miles longer than the Grand Union Canal route.

Competitor Ben Saunders, 26, a polar explorer based in Parsons Green, said: "A lot of people have been asking me if I'm insane to do this, but I just want to see if it can be done. It's nice to be doing something closer to home, although 10 marathons is daunting.

"I'm confident I'll finish, but sleep deprivation might be a problem. It's also going to be quite warm, so for a polar explorer that might be a problem. I will be trying to sleep on the course though."

The Thames Ring 250 will start and end in Streatley-on-Thames, taking in some of the most tranquil stretches of the river including glimpses of Richmond, Milton Keynes and Oxford, and consisting mainly of towpaths.

The route is based on the Thames Ring popular with boats - which normally takes around three weeks to complete. Competitors will even use the water taps normally for tourist boats to refill their water bottles.

Organiser Anthony Taylor said: "This is the longest non-stop race ever held in the UK, and we are all intrigued to see how people approach it.

"We heard about the boating route, and started to wonder if it could be run. We put out an email to the ultrarunning community, and we now have 36 competitors, including people from the US and across Europe."

The route will be split roughly into 10 marathon-sized sections with rest and food stops between each. Runners will have a small backpack containing water, food and a mobile phone for emergencies.

Checkpoints will also offer food including rice pudding, marmite and jam sandwiches, sweets, Jaffa cakes, crisps and fruit.

Mr Taylor said: "The biggest problems competitors will face are dehydration, blisters and exhaustion. Sleep will also be critical, and we will be providing tents at each checkpoint.

"We are not really sure how long it will take, although we have a cut-off time of 100 hours. We'll be manning the finish line after 60 hours." Mr Taylor said he expected competitors to take breaks of between three and four hours if they need sleep.

"Catnaps of 15-20 minutes may help, but I think people will opt for longer stints. However, we don't know how people will approach it, and it is possible someone will attempt it without sleep, although that seems unlikely."

All the runners are believed to be experienced. Mr Saunders, who became the third person in history to reach the North Pole solo and on foot in 2004, said: "I'm quite fascinated by endurance and how far we can push our bodies."

Start: Streatley 10am today
Checkpoints roughly every 25 miles offer food and tents for sleeping