By Tom Fordyce

Paul Selby prepares for his 597th mile

It is day 26 of the Flora 1,000 Mile Challenge, the sun is shining, and the blisters are looking ugly.

We are in Rotherhide, east London, on board possibly the healthiest tour bus of all time. It is home to five people who have agreed to run 1,000 miles in 1,000 consecutive hours - at the rate of a mile per hour, and no more - up and down the course of the London Marathon.

Remarkably, considering that none of them have slept for longer than an hour at one time for nearly four weeks, the mood is chipper.

Only one man has dropped out - Lloyd Scott, the man who did last year's marathon in a Victorian diving suit - and that was for personal reasons.

Feature: The 1,000 Mile Challenge explained

The rest look no more tired than you or I after a night on the sauce.

"I've been taking it a day at a time," says Paul Selby, at 56 the oldest of the runners. "We'll be reaching the 600-mile mark today, and that's a good landmark. But I fear there is going to be a twist in the tale. The worst could still be yet to come."

It could be just around the corner. 607 miles was the point where Captain Barclay, who 200 years ago took on this crazy challenge as a bet, had to be whipped awake and onwards by his pals, so exhausted was he.

Barclay was also forced to carry pistols with him to ward off the rascals who were trying to nobble him. The difficulties this time around have been more prosaic - where to go to the toilet, when you are only allowed to pee in the one on the bus, and where to shower.

THE ORIGINAL NUTTER Name: Captain Robert Barclay Allardice Born: 1779 Attempted: 1,000 miles in 1,000 hours At: Newmarket Heath Won: 16,000 guineas

Fire stations and rugby clubs have come to the rescue, but it isn't always possible to co-ordinate your bowel movements with a handy convenience, particularly when your metabolism has been knocked bandy by sleep deprivation and extreme levels of exercise.

"I had to choose yesterday between a massage and shower," says Selby, "because there wasn't time in my rest break for both.

"I'm also worried what happens when the weather gets worse. When the sunshine goes and it starts slashing down, we'll all be miserable as sin."

Conditions on board the bus are cramped. Each runner has a small bunk to sleep on, with storage room above and below.

There is a tiny lounge area, a kettle, microwave and fridge and energy bars and drinks of all shapes and sizes.

"The bus is like the Tardis, except the other way round," says Shona Crombie-Hicks, an experienced marathon runner who is one of two women taking part. "It looks huge from the outside, but when you get in there..."

Fancy a blister like this on your foot?

Crombie-Hicks is also the one to suffer most from blisters. At the request of the bloodthirsty BBC, she peels away the dressing to reveal a red mess where her heel should be.

"It was the hardest part of the first week," she says. "The blisters were so debilitating I couldn't run."

And run she must - for when these plucky souls complete their 1,000 miles, they must then do the actual London Marathon. As a race, against each other.

In practical terms, this means they face the unthinkable - attempting to train for a marathon while simultaneously attempting the 1,000 Mile Challenge.

Something to think about the next time you moan about having to walk back from the shops, isn't it?