THE Royal Marines’ fabled Endurance Course doesn’t take place at their Lympstone base. It’s on nearby Woodbury Common.

As we travel there by van, PT instructor Sgt Bart Ledger and my Royal Marines minders, Neil Hipkins and Ben Wright, keep reminding me this is the course with the infamous “sheep dip”, where you get shoved under water through a boggy tunnel. “Practice holding your breath for 15 seconds,” Bart advises, firing up his enormous stopwatch. As the young folk say, OMG!

For recruits, Endurance is timed and completed with the crushing weight of a backpack and rifle, with which they have to gallop back to Lympstone and hit six out of ten rounds on a target.

For me, it all starts with a couple of circuits of the car park, wearing a belt comprising 21lb of weights. After three laps I feel like a human tent peg. The weight is actually driving me into the ground.

We set off at a gentle jog, mercifully minus the weights. I run most days but never in army boots, a rugby shirt and fatigues. Halfway up the second hill, disaster strikes. I get what I very much fear is my first asthma attack. “Breathe in through your nose and out through your mouth,” advises Neil.

Back on the flat, Ben bounds, gazelle-like, along the stream while I splash behind like Nellie the Elephant. Eventually, at a muddy bend, we stop.

“Right,” says Bart. “Lie down there and do the leopard crawl.” This is that thing you see Royal Marines doing close to the ground, scrabbling forwards on their stomachs. I just scrabble. “Your back’s too high!” says Bart. I thrash around, barely shifting, and have never felt such a dork in my life.

We set off again and with a soaked jacket and boots filled with water, I’m even slower.

Eventually we arrive at a narrow pipe filled with stones and what looks like half the River Exe flowing through it. In combat, Bart explains, Royal Marines sometimes have to escape through sewers. I crawl through as best I can, feeling like a contestant in I’m a Celebrity… and praying I’m not savaged by rats.

Outside again we wade through sludge, we slither down hills.

I tackle two more tunnels, one which is pitch black and dry. Desperate not to wimp out I close my eyes and feel my way. I’m concentrating so hard on trying to get through, I forget about rats and snakes.

We arrive at a small river. You wouldn’t think wading waist deep, fully clothed, through a small river would be so pleasant but on this boiling hot day it actually feels nice. Until I try to run on the other side, that is. Weighed down with mud and water, my legs will hardly move.

I ask Bart how badly I’m doing. He won’t be drawn, kindly pointing out more than 99 per cent of people in Britain will never even have had a go at this course.

A quick slide down a rain-runnel and we arrive at the infamous sheep-dip. I want to die... of embarrassment.

As Royal Marines will, they’ve been having me on all the time – the thing is no longer than my kitchen table and the water is so clear you can see the bottom!

Bart explains: “Ben will shove you through and Neil will pull you out.” Bart stands by with a special tool to open the outlet valve, should I get stuck and be in danger of drowning. But we don’t need it. The whole thing takes three seconds and despite the fact I now look like a drain, the feeling of achievement is incredible.

We’re supposed to run back but my legs and lungs won’t let me. So we walk and talk.

Five of Bart’s colleagues have been killed in combat. Neil has already done three tours of duty in Afghanistan. They only tell me this because I’ve asked them. Royal Marines don’t dwell on negatives – they can’t afford to. As I’ve learned, they’re brave and decent and above all, practical. “Everyone has to die,” says Bart. “I’d rather die for something rather than nothing, like a heart attack.”

When I leave, Bart presents me with some Royal Marines goodies, including their dagger lapel badge. I will wear mine with so much pride because, now I know a tiny bit of what it’s like to be one, I feel humbled and awed by their spirit.

You may wonder, of course, why I needed to find out exactly what it’s like to be a Royal Marine. Well, it’s to prepare for a very special mission, more of which later…