I’m lying in the mud, full of fear. We could be going over the top at any minute. I look to my left and see 45 and 39. They’re just as scared as I am. Then the command goes out: “DELTA TEAM, MOVE!”

We scramble to our feet as fast as we can, run as fast as we can. There could be an attack at any – “GRENADE!” We throw ourselves down as fast as possible – 45 manages to penguin slide across the grass. “CHARLIE TEAM, MOVE!” The second unit gets up and sprints across the torn up turf we’ve left behind us, trying to catch us up before (“GRENADE!” ) they have to fling themselves back to the ground. it’s 7.55pm, and war has broken out on Peckham Rye park. If we make it back alive, I promise, I’m going to be a changed man.

Tonight’s session had been particularly tough. We’d given our team-mates piggyback rides. We’d crab walked across the rugby pitch. We’d even dragged our team-mates half way across the field, trying to work out what the correct etiquette for hand placement was when dragging a team-mate. But this grenade business made the rest of it look, well, like a crab walk in the park.

Two teams, each trying to make it back to base camp alive. Alternating between running and diving between freshly laid dog egg mines. In the rain. In the mud. Not a pretty sight. It was surprising, because the emphasis in British Military Fitness is always firmly on the fitness and almost non-existent on the military. And yet here we were, running for our lives. We eventually made it  back with few (well, no actually) injuries. We’d beaten the enemy and their invisible grenades, and had the grass stain medals to prove it. A brilliant victory then. But if you ever meet me, and see that thousand yard stare in my eyes, you’ll know why.