Have you ever reached your anaerobic threshold? This is the point where your body stops burning oxygen efficiently enough to keep up with the demand for energy. The result, in those unused to pushing themselves to this point, is cramp, desperation and an overwhelming desire to lie down. It's the level sprinters such as Usain Bolt work at. It's the level locals in Chamonix work at when they go to buy the morning loaf. It is not, it transpires, one I work at.

"But hey, that's fine," chirped Julia as she prodded my twitching form with a Nordic walking pole. "There's no judgment here – we're just keen to show runners they can do it." Gently expire within view of Mont Blanc? Sure can.

If it's a sporting challenge you're after, Chamonix is your place. Pushing limits is its lifeblood. So if you've cracked that 10km road race barrier, or you've run all the trails the British Isles has to offer, the next stop is the Mont Blanc massif, a place where eye-crushing mountains dare you to set foot on their flanks. This is, after all, the town that hosts the Ultra-Trail du Mont-Blanc, an extreme trail race where participants cover 103 miles with 9,400 metres of height gain. Non-stop. During her week-long trail running camp, Julia Tregaskis-Allen of Tracks and Trails has made it her mission to take daunted, determined and off-road-curious runners and open up her Chamonix playground to them.

I was keen. I've run a marathon and despite taking about six months to recover I've kept up the running over the years and I like going off-road whenever the chance arises. My trainers have Gore-Tex uppers for all those puddles I charge through. I was ready.

It took a day to crush me.

By the time we had done our first gentle run from our base at Chalet Maverick in Les Bossons up to the Christ Roi statue in Les Houches – a distance of about three miles – I couldn't feel my legs. Or lungs. It was a quick lesson in how trails in Chamonix have a tendency to go uphill or downhill very steeply but rarely along the flat.

"Your body can be accustomed to different types of running," said Julia's partner in crime, personal trainer Steph Lightfoot. "Because of the nature of trail running you're always changing your stride and motion, dodging obstacles. Being able to pound out the miles on the road is great, but those distances don't automatically translate to trail as there's so much more to it."

I rested my back on the cool stone of the 25m high statue and peered through eyes stinging with sweat at Mont Blanc and the Glacier des Bossons, shattered hunks of blue ice hanging off the mountain in suspended animation.

"Let's just enjoy running and drop all the negativity, shall we?" said Julia. "Trails can make your running far more relaxing because you're not thinking of miles and timings. Look at the scenery here, enjoy the terrain."

She handed me some Nordic poles, slightly thinner and longer than a ski pole and useful for stabilising your descents and engaging your arms more efficiently in propelling your body uphill. With a deep breath I hauled myself to my feet and we were off again, the poles not quite stopping me from running like a troll but certainly adding a new dimension to proceedings that detracted from the lactic acid build-up in my side. Leaping over trees recently felled by gale force winds, crossing glacial run-off rivers and hopping around fallen boulders also helped.

On arrival back at Maverick, nutritionist Polly Grosset had a perfectly balanced, healthy, protein-rich meal waiting – these seven days take a holistic approach to health and fitness, rather than stuffing you with creamy tartiflette and red wine, which isn't the best recovery meal apparently.

Running in the mountains and away from the road isn't simply about putting one foot in front of the other. It's about kit and preparation, route planning and first aid knowledge, stretching properly before and after and having a good understanding of yourself and your capabilities. All this we covered. Out here the stakes are high – a twisted ankle could lead to a really bad situation. Julia handed me a compass as I was dreamily staring at the outdoor hot tub, and asked me to take a grid reference on the well-used map of the valley spread out on the dining table.

No chance. She looked at me with resigned patience before beginning at the beginning. We spent hours learning how to take our bearings, how to read contours and how to engage Naismith's rule in order to get a rough idea of how long it would take us to complete a certain route: allow one hour for every 5km (about three miles) on the flat plus one hour for every 600 metres of climbing.

"It's best to plan conservatively – don't push it,'" said Julia. "A lot of it is endurance – anyone can do it. You can be a trail runner for life because it doesn't have the same wear and tear on your body." After days of running high around the valley and into Switzerland we had stretching sessions at Chamonix Gym with Steph, a woman whose knowledge of anatomy and physiology, nutrition and how to maximise training had us stretching muscles we didn't even know were compressed.

We did technique classes and running drills, core stability work and flexibility classes. Steph gave us each a one-hour fitness assessment, the best part of which involved sleeping on her massage table to test our resting heart rate. The worst part was the bleep test to measure our cardiovascular integrity. I don't think we need to dwell on the integrity of mine – or lack of – here. Safe to say I scored highly in the resting section.

In a week we covered the course of the Chamonix half-marathon, also known as the Chamonix Cross, a truly inspiring circumnavigation of the valley which at points has you far away from the valley floor and running close to the sky. One day we ran 10 miles and it felt good – unthinkable at the week's start.

There were times, as I ran from Le Buet to Chamonix on a route packed with glorious nuggets of history – such as the fact that the Hotel du Buet on the Swiss border, where we began our route, hid Jews in their cellar during the second world war and helped them escape to Switzerland despite also being used as a Nazi HQ – when I had to shake my head and laugh, such was the great feeling of coursing through such an extraordinary environment powered only by my newly enthusiastic – and properly fuelled – body.

Obviously I'll be leaving the Ultra Trail for another year. But perhaps it's not quite such a distant prospect anymore.

• Tracks and Trail (020-8144 6442, tracks-and-trails.com) provided the trip. Its next six-day trail running camp with availability starts on 23 September and costs £1,095, including accommodation, coaching and food. Swiss International Air Lines (Swiss.com) flies from Heathrow to Geneva on the above date from £108 return. Airport transfers from around €26