Thundering along a springy turf track at a turbo-charged gallop, I realise I'm not quite in control. I concentrate on keeping my feet in the stirrups and steering a safe path over the heather-strewn landscape. "Oh please God, don't let me fall off," I whisper as we fly across the hill.
Poppy, the pretty Welsh pony I've been given for the weekend, is on her way home. We've spent a blissful two days exploring the Radnorshire hills, but while I'd happily stay several more days, Poppy can smell the bucket of food awaiting her back at the stables.
Just as I'm starting to get a bit panicky, she lollops back to a walk; she knows that it's time to consult the map. There's a choice of routes ahead and Poppy remembers that she needs to give me time to think.
This is unguided trail riding – unpredictable, utterly liberating and occasionally a bit scary. It's a world away from the dull, organised plodding that I know as pony trekking.
"It's the difference between an afternoon at a dry ski slope and a fortnight in the Alps," says Matt Williams, who runs FreeRein, the outfit in the Welsh borders that has organised my trip.
To take part you should be a competent rider and know how to tack up but you don't need to be an expert. Guests are released on to the hills with ponies, maps, directions and a promise that dinner, bed and their luggage will await them in an inn or B&B later that day. And that's it.
There's no comforting guide to open and shut gates, show you the way and see you past whatever animals or obstacles you may encounter. There's nobody to sort your horse out for you in the evening. There's not even a standard route. The FreeRein maps offer a variety of suggested ways to get to your destination, but how you get there is up to you. In a world of high-visibility vests and health and safety constraints, this level of freedom seems almost too good to be true.
If it all sounds a touch reckless, though, it's not. Matt and his father, Will, run an impressively tight ship. As soon as I arrive at the FreeRein HQ in Clyro with my friend and riding companion, Julia, our overnight bags are labelled and spirited away, and tea and coffee produced.
A charming Czech helper called Tomas introduces us to our ponies – kind-faced Welsh cobs which, like all the other horses we see there, are in good condition and obviously well cared for. "They're both very forward going," says Tomas cheerfully as we groom and tack up our new friends. "Especially Poppy."
I look at him anxiously. Is "forward going" horsey code for uncontrollable?
I needn't worry. For all the freedom, safety is a big priority here. All the FreeRein mounts are subjected to months of training to ensure they're reliable and steady. They have to get on with the other horses and have a temperament that lets them take traffic, tractors and even paragliders in their stride.
Sure enough, our ponies are remarkably calm. They don't bat an eyelid when motorbike scramblers appear from nowhere, nor when frenzied collies dart out from behind a tractor. Of course they're not robots – they're startled when a pheasant flaps noisily out of a hedgerow, and refuse to pass a waving line of washing until we've reassured them – but they're as calm as horses ever can be.
Within half an hour of setting out, we've reached the Begwns – a National Trust-owned stretch of heath with wide grassy tracks. The ponies break into an effortless canter and we thrill to the feel of the wind in our hair.
We stop to consult our maps and I wish, briefly, that I'd listened more carefully in geography. I know that lines mean hills, but beyond that I'm a tragic caricature of a female map-reader. Luckily, the directions step in where my knowledge runs out and we turn off on to a steep, stony track, hoping that it will take us to the village of Painscastle. I'm keen to visit this former droving centre. At its height it boasted five inns, and like the cattle drovers before us, we're after somewhere to quench our thirst and tether our beasts.
Happily, we're on the right track and are soon settled in the Roast Ox Inn. In days gone by we'd have paid 1/2d to keep our animals in Halfpenny Field opposite the pub. Now there's no charge for parking and, to our delight, we find more horses than cars in the pub's car park.
The afternoon takes us into wilder parts and we barely see another soul. We ride past streams and across fords, though farmyards and over hilltops. We pass endless fields of sheep with newborn lambs staggering and skittering beside them. Stopping on a hillside, we find ourselves eye-to-eye with a red kite, dipping and bobbing on a thermal. Its tail feathers, moulded into the characteristic fork, glow a rich, burnished orange in the afternoon sunlight.
Soon the rhythm of the horses has lulled us into a state of gentle relaxation and we forget to chat. Almost in a trance, we make our long descent down bracken-fringed paths into the village of Rhulen where we're spending the night.
Cwmfillo is just the sort of place you want to be after a hard day in the saddle – a warm, comfortable farmhouse with soft beds and a friendly old sheepdog who nestles by your feet. Our hostess, Julie, greets our ponies as enthusiastically as us. The FreeRein boys have been there already and we find buckets of feed and grooming brushes laid out in the stables. With the horses safely turned out into a field for the night, I sink into a hot bath and watch a pair of buzzards wheeling above a hillside dotted with sheep and ponies.
A combination of three courses of home cooking, six hours in the saddle and a silent, starry night make us speechless with exhaustion by mid-evening. I sleep for a solid 10 hours and wake thoroughly restored and ready for the off.
Our route back takes us on a different, equally spectacular, route past rocky outcrops and glassy pools to Llanbedr Hill. From the top, we look out over a landscape with neither a house nor a mobile phone mast to tell us what century we're in.
Skylarks rise from the bracken as we saunter past and buzzards trace a lazy circle off in the distance. Closer by, wild ponies raise their heads to watch our progress, then continue grazing.
As the track turns into tarmac, we're on course for another lunch in Painscastle. This time, over a glass of cider and a herby pea soup, we are regaled with scurrilous gossip and unrepeatable tales of intrigue by a wily old local with a smile as wicked as the stories he tells.
The last afternoon sees the ponies pick up their pace. We don't share their eagerness to get back – the Sunday afternoon feeling is heightened by the knowledge that we've discovered a way of life which has fast become addictive. We could happily have been drovers, we agree, picturing the romance of long days on horseback and longer nights in ale-soaked inns.
For all our sadness at coming home, our return is a triumphant one. We managed not to get lost and I survived Poppy's attempt at a 0-60 record – both good reasons to celebrate. We make our way along the FreeRein drive with our heads held high.
Days later, my still burning muscles are enough to convince me that I'm probably not drover material after all, but it's been a happy fantasy – a kind of equine Brief Encounter. I'm already working on the sequel.
• A two-day ride, with luggage delivered to your accommodation en route, £298. FreeRein (01497 821 356; free-rein.co.uk) offers unguided trips up to a week long, as well as a range of guided trips.
