Dan Rivers reports from Morocco as Afghanistan veteran Duncan Slater closes in on history in the "toughest race on Earth" across the Sahara desert.
Imagine the interminable length of time it takes to fly from London to Singapore. Then double it. That's how long Duncan Slater has been walking.
Thirty gruelling hours would be bad enough over 86 km (53 miles), but how about in temperatures touching 50C up and down sand dunes and gullies hundreds of metres high?
Imagine doing that after you’ve already done three back-to-back days of marathon walking in identical terrain - and all on prosthetic legs.
You begin to have the measure of Duncan’s remarkable achievement.
The feared stage four of the Marathon des Sables is over and 37 people have already dropped out.
It is testament to Duncan’s iron will that, despite a hamstring injury and total exhaustion, he is carrying on.
He is now focused on the final stage, a marathon distance final push.
His wife Kim has been monitoring his progress both through our reports, tweets and the MDS tracking software.
She is convinced "he’s got this" and I agree.
This time last year his stumps were rubbed raw and on medical advice he was forced to withdraw.
This time, thanks to the brilliant new prosthetic legs he had made in Italy, he is in much better condition.
His hamstring is sore and he is obviously drained, but I get the feeling now he can begin to dream about the end.
We’ll be there to watch history being made.
You can donate to Duncan's chosen charity, Walking with the Wounded, by visiting his fundraising page