It's late afternoon and the Irish Sea is creeping up Cumbria's Silecroft beach as anglers prepare lines for an evening spent in search of bass and flatties. The sand and shingle shoreline extends as far as the eye can see.

Except suddenly I can't – see, that is – for great clods of flying sand. As I'm at a flat-out gallop on a 17.2 hands high (hh) Clydesdale weighing almost a tonne, the effect is slightly disconcerting. Fingers woven tightly through Ben's dense mane, I utter a teeny prayer: "Please God, don't let me total a kitesurfer."

I'm just hours into a six-day, 100-mile group trek through the south-west corner of the Lake District national park with Cumbrian Heavy Horses, but already I've learned something: there's having sand kicked in your face … and then there's having sand kicked in your face by horses with hooves like hubcaps doing 25mph. Behind is not the best place to be.

Fortunately, the storm clears as swiftly as it started (jeez, heavy horses start swiftly), leaving in its wake eight hirsute giants – three Shires, one Franco-Belgian Ardennes and four Clydesdales, the tallest of which stands at 18.3hh – seven sandblasted guests and ride leader Andy.

"Didn't I say trot to start?" he sighs, with just a hint of Disappointed Dad. "But we have to beat the tide to the headland if we're to reach Gutterby tonight, so let's crack on. This time with control, eh?"

My heart's still racing from that first, almighty power surge, but Ben is raring to go, dancing on the spot like a Brobdingnagian rocking horse.

Over the centuries the English Shire and Scottish Clydesdale have hauled everything from coal wagons to first world war gun carriages, but since the mid-1900s their numbers have plummeted. Viewed solely as draft horses, both are now deemed rare breeds. But here's the twist: as fit and fast as three-day eventers and, despite those enormous hooves, as nimble as goats, these are no one-trick ponies but – as I'm to discover – two of the greatest, most unsung riding breeds.

This is a conviction shared by all who work or, like Andy, volunteer at one of Britain's only specialist heavy horse riding centres: Cumbrian Heavy Horses (CHH). Head of procurement for an NHS Trust, Andy escapes to CHH most weekends, kipping in the barn so as to spend every waking minute with the horses he adores.

"Sometimes I surprise myself by how much it means to me to be able to do this," he grins. "Fortunately, I have an understanding wife."

Our days start early; guests are encouraged to double as grooms – though it's not compulsory. We're a mixed bunch, from 18 to late fifties, from as far apart as Hong Kong and the Netherlands, but everyone mucks in. These horses are nicknamed gentle giants with good reason. Only when the sunscreen comes out does Ben forget his manners and use his size to his advantage. Slathering his pale nose, bobbing way above my own, requires cunning … and a bucket to stand on.

From Gutterby we weave along Black Combe's bracken-clad shoulder, following narrow trails forged by white-faced Herdwick sheep, through Ravenglass and on to Muncaster. Like the circus that's come to town, we're quite the sensation. Gobsmacked kids rush to greet us. Everyone wants to know where we're heading – as well as "How do you get on?" There's only one dissenting voice. Hearing hooves, a yappy mongrel belts from its house, then skids to a silent halt – mouth open, mid-bark – in sheer astonishment at the monsters it had thought to see off.

Accommodation over the week includes chalet cabins, a camping barn and Eskdale's 18th-century Penny Hill Farm – once owned by Beatrix Potter – where chickens career across cobbles and orphaned lambs clamber to suckle from buckets with teats. Breakfast, eaten under the gaze of a stuffed red squirrel (Nutkin?), is cracking. Sharing a bathroom between seven (a problem now rectified for future rides) is less so – and I find fellow guest Tim in his boxers resorting to a bucket scrub down in the yard. Tonight, though, we're lording it with en suites aplenty at the Muncaster Country Guest House (muncastercountryguesthouse.com). Shattered but showered, we collapse with G&Ts and coo over wee lambs gambolling in the next field, before devouring hearty Cumbrian Tattie Pot and velvety lamb shanks.

Ride leaders switch every two days, so Muncaster sees a blinking-back-tears Andy ("I wish I didn't have to work – I'm so jealous") replaced by fellow volunteer Beverley, who runs a needlecraft business but helps out "to keep sane". Higher than the hedgerows, nose-to-nose with butterflies, we ride out over Muncaster Fell and through bilberry-carpeted Miterdale Forest where, given the low branches/high horses combo, limbo skills are a definite plus. Then we're off across barren, boggy Irton Fell to Nether Wasdale. Tonight's stop – the bustling Strands Hotel (strandshotel.com) – has its own microbrewery. "I think my mouth's been anaesthetised," mumbles Tim, two pints of "Irresponsibly" later.

But it's from the next morning that things get truly wild. Dwarfed by the mountains Red Pike, Kirk Fell, Great Gable and Scafell Pike (England's highest), we ride into savagely beautiful Wasdale, following the steely shoreline of England's deepest lake, Wastwater, horses pawing eagerly at the shallows. And then it's up, up, up as we skirt Scafell's base (ankles burning as we stand in the stirrups, jockey-style, to lessen the horses' load) and trek on past Burnmoor Tarn.

This is a hauntingly desolate place of scrubby grassland, granite and 500m-year-old volcanic rock; we're alone bar grazing Herdwicks and whatever is beneath the funerary cairns of the five Neolithic stone circles we pass.

The horses make light work of what is, at times, treacherous terrain, especially on our final full day on the fells. Led by staffer Kirsty (another atypical instructor, on a break before a neuroscience MSc), we begin our ascent of Hardknott Pass. With a one-in-three gradient, vertiginous Hardknott vies for the title of England's steepest road – not that the horses care. Cutting across hairpins, we overtake a blistered cyclist, pushing his bike in socked feet, who begs for a backy, and a car with Dutch plates, the acrid stench of its burnt-out clutch a warning to all (not just motorists from flat countries).

The view from the crest is immense, encompassing our route so far and stretching beyond Harter and Birker Fells to the coast; the Isle of Man is a dusty smudge on the horizon. But as with all roller coaster rides, it's the descents – ludicrously steep mountain tracks combining shifting scree, potentially leg-breaking boulders and huge drops – that are the real rush. Over the week I learn to trust Ben and give him his head, but as we shuffle down from Wallowbarrow Crag, Ben's colossal hooves delicately picking their way through sheer rubble, Kirsty admits: "The first time I walked this track I remember thinking I'd never bring a horse here – but that was before I met this lot."

And then it hits me: we haven't seen another rider since we were on the beach. These incredible heavies have taken us places no other horse I've ridden, let alone my own legs, could have reached. To boldly go where no horse has gone before? Maybe. One thing's for sure, though, this is star trekking of the highest calibre.

Now if someone could just beam me back up …

• The next seven-night Cumbrian Classic heavy horse trip is on 11 May, with six more dates over the summer (01229 777764, cumbrianheavyhorses.com, £1,500pp full-board). Participants must be ride-fit and capable of riding at all paces. Abigail brushed up her skills before the trek at Stag Lodge Stables at Richmond Park (020-8974 6066, ridinginlondon.com). Virgin Trains (0871 977 4222, virgintrains.com) has returns from Euston to Lancaster from £71. Northern Rail (northernrail.org) open returns from Lancaster to Green Road, the nearest station to the stables, cost from £15.20. For more information on the Lake District visit golakes.co.uk and enjoyengland.com

• This article was amended on 11 April 2012 to correct the sentence which stated that Cumbrian Heavy Horses was Britain's only specialist heavy horse riding centre. Adventure Clydesdale (adventureclydesdale.com) also runs trail rides on Clydesdales in Cornwall