It's a motley crew that assembles around the big, red 4x4 in the carpark of the Nest hotel in Ngong, just north of Nairobi. The majority of the 40 or so people are overweight. A few are drinking fizzy drinks. One lady, with a face like a snarling dog, is smoking a cigarette. Everyone is wearing running kit.

"Today's long route is 10km," says the man in charge. "The short route is 8km. Enjoy."

Most of the people here look like they'd struggle to make it up the stairs to the bar. But with good natured smiles and jokes, we all file out of the carpark, on to the road, and start jogging.

After two months training with elite Kenyan athletes up in the Rift Valley, I've come down into town, to Nairobi, where the running scene is more varied.

Nearby, up in the Ngong Hills, there are still plenty of serious athletes to be found, but today I'm running with the infamous Hash House Harriers.

With over 1,700 groups meeting in most major cities around the world, the HHH is an international phenomenon. More a social club than a typical running club – they like to describe themselves as "a drinking club with a running problem" – they nevertheless head out on regular long runs all across their respective cities.

After all the hard running I've been doing in Iten, struggling along at the back of every group I join, this, I hope, should provide some light relief.

Rather than run along a set route, the Hashers follow a trail marked out in advance with white chalk scattered on the ground.

The pace is excruciatingly slow at the back of the group as runners heave themselves along the road, almost being knocked over by buses crammed full of commuters.

At the head of the group, a few lean runners are getting away. I chase after them, but as soon as I catch them, they step behind a wall and stop.

They're all grinning.

"What's going on?"

One of them, an elderly man with one of his front teeth missing, points at two white chalk lines on the ground.

"That means it's a false trail," he says. But he doesn't want the others to realise, at least not until they too have come all the way down the dusty side road as we have.

This is not going to be a normal run, I realise.

Once we get back on track, returning en masse to the main road and taking a different chalk-marked side road, the same few runners hurtle off at the front again, and I stick with them. We soon find ourselves running through the back yards of some collapsing wooden houses, ducking under washing lines, leaping over small children playing in the mud.

But we seem to have lost the trail. As we stand around deliberating, a man in a doorway points down a narrow gap between two of the houses. Without thanking him, we rush down it, and sure enough, there are more chalk marks.

"On, on," the others shout at the top of their voices, as the slower runners begin to catch up. Children stand and watch us pass, too bemused even to make a comment.

And so it goes on. Every time we get stuck at a turning, it gives the other runners a chance to catch up. When we find the right way, we yell "on, on" and the charge resumes.

Despite having initial reservations, I'm finding it all quite exhilarating. We're running like loonies through tumble-down back streets, looking for white chalk marks. I even find myself yelling out when I find one.

"On, on," I yell. Two women sit in a doorway watching me run by. Behind comes a long line of plodding Kenyans in tracksuits and fluorescent bibs.

At about halfway we find a car parked with the boot open. Inside are cups of water, slices of melon and chunks of sugarcane to suck on. Sitting in the front of the car is the woman who was smoking at the start.

I'm one of the first to arrive, but soon everyone has caught up. As we stand around eating and getting our breath back, someone says: "Let's have a song."

Spontaneously, they all break into a hearty version of Singing in the Rain, except with compulsory actions like wiggling bums and sticking out tongues. The people living down this particular backstreet, with its dusty hair salons and mango stalls, stand around in groups, agog.

Although there are a few other wazungu (white people) in the group, the Hash is mainly made up of Kenyans. They all drive big cars and are more than happy to hand over KSH150 (£1.10) just to run – more than many people here in Kenya earn in a day. Afterwards, they drink the night away, with beers at specially reduced prices and rooms booked at the hotel for those too drunk to get home.

In a country full of super athletes driven by poverty, it is among the well-educated, overfed rich that I have finally managed to find some Kenyan runners slower than me.

I enjoy the brief glory of being the first Hasher to finish the course. Next week I'll be back in Iten, in my customary role, as the slowest runner in town.

• The book Running with the Kenyans by Adharanand Finn will be published in 2012