When I signed up for the first ever Zombie Evacuation 5k run, I was disappointed by the wimpy reactions of my friends. One said: "Hasn't it ever occurred to you that this is the perfect cover-up for a real zombie invasion?" Another asked: "Aren't you worried you'll get so caught up in the experience you'll end up battering someone to death?" No, no, I sighed, it's just a run with some silly folk in fake blood to add to the fun.

I would now like to take the opportunity to apologise to these sage friends with their sense of foreboding: reader, it was terrifying.

The race was held across country in Royston, and began with queueing in biting winds on one of East Anglia's many flat, exposed fields to collect GPS trackers and a clip-on belt. The belt had three green tags Velcroed to it, representing our 'lives', which the 'zombies' would lunge for as we ran (health and safety forbidding them to rugby-tackle us, unlike in the promotional video for the event, which I spent more time than I care to admit watching).

Assembling at the starting line – already at the back, and somehow managing to slip on some mud before I'd even thought about running – I reminded myself that there was nothing the zombies could do to me, other than a bit of growling and jostling. Yet, as the 'soldiers' who were supposedly leading us to uninfected ground shouted at us to start, and the first gag-inducing smoke bombs were released, something very primitive inside me kicked in.

I had a Lion King moment, hurtling like a distressed wildebeest along a dirt track and swerving into a ditch to avoid a grasping arm. All I could hear were shouts, grunts and the occasional gunshot from our uniformed defenders. But come the real apocalypse I won't be following military advice: at one point I was instructed to head into a pack of three ravenous zombies, rather than follow an ingenious short cut I'd discovered across some farmland.

I quickly realised that screaming and running are incompatible, and that I would have to choose between the two. The fact that the following morning I had no voice left should identify which I plumped for.

There was then a mercifully quiet fieldside track, where we could regroup and catch our breath before plunging into the woods. I got off to a poor start, as a middle-aged zombie made a beeline (z-line?) straight for me. Despite the ridiculousness of the situation, I felt a surge of pure adrenaline which accounted for some poor decision making: I hurtled across some slippery logs and straight into the path of a stethoscope-clad, blood-splattered doctor, who seemed almost as surprised at me. As much as it pains me to admit it, he actually laughed at my panic, as it was clearly too early in the race for anyone to be quite so bad at surviving.

After that, the fear took over. The worst part was running through a section of the wood where nothing happened at all - apart from in my imagination. My low levels of fitness and general mistrust in my body's ability to keep me from harm didn't exactly help. By the time we came across the next batch of groaning, limp-limbed creatures, I was an emotional wreck and ploughed right into them, face contorted in fear.

I was two tags down by this point, and having elbowed my way through an 'infected hospital' (military netting and plastic body parts), I realised I faced a gladiatorial gauntlet of reanimated corpses, and promptly suffered a minor mental breakdown. I always assumed that fear led to a fight or flight mentality, but it turns out there is a perfectly decent third option: surrender.

I'm not proud of this, but it all ended when I shrieked "Just take it!" at a surprised-looking zombie, who, with the greatest reluctance, removed my final tag. Laughing maniacally as I realised I was free, I walked past the remaining undead shouting, "I'm one of you, you can't touch me now!"

I learned far more about myself that I ever wanted to know in the course of a mere five kilometres: namely that in the event of a real apocalypse I should shoot myself in the head immediately. Also, I discovered that everyone else is much braver than me: my shaky route back to the safety of the car took me past laughing groups of friends, couples and even children, blithely talking about signing up for next year.

As I stripped off my zombie T-shirt and 'infected' dog tag and stepped into the shower, I knew I would be forever haunted by a ringing in my ears: the sound of zombies laughing.

• The next Zombie Evacuation is on 11 November at Pippingford Park, south London. For more information, visit zombieevacuation.com.