I'm waist-deep in gravelly mud that somehow manages to be sticky, slippery, and sharp, all at the same time. I flop onto a ledge like a seal emerging from the sea, roll gracelessly to my knees, stagger over the wall of earth and bum-slide into the muddy trench on the other side. That's five down and what looks like another twenty to go.
My usual answer is "I'm doing this because everyone else is". But there's no one else here. It starts to feel pointless.
My backup answer is "because I said I would", and that gets me through. But not quickly. I've entered the last wave of Nuclear Rush - the only wave that didn't sell out - alone, and for long periods I'm the only runner I can see.
I'm surprised by how much difference that makes. Far from speeding me up - I have an unobstructed run at every obstacle, with none of the momentum-sapping queues at other races - it's actually slowing me down. Even at the very end, my final approach is more of a hero-walk (imaginary explosions bringing down the course behind me) than a triumphant finishing sprint.
I've done some serious races. I like to imagine that I'm tough, independent, rugged. That's one of the reasons I do these things at all. But it's just not true. As it turns out, physical performance depends not only on the obvious creatine and glucose, nor even the slightly less obvious psychological factors, but on social factors as well.
Maybe that's only news to me. I've always thought of running as a solo sport. I love the isolation of long-distance training - the experience of being three hours into a four hour loop, headphone batteries long dead, the last familiar landmark miles away, and nothing but footsteps and thoughts for company.
But I do get the start-line buzz, that incredible energy of being surrounded by hundreds of people who are - I've finally realised - my tribe. Ultimately, they might call it a "race", but we don't run to beat each other.
We run to be with each other.