As we followed the river downstream, it broadened into a stony eroded bed, down which the water found its swift-flowing route past little hamlets in their chocolate-box settings beneath forest-clad slopes and snow-tipped mountains.

Choughs riding a thermal

The valley again narrowed to form a small gorge, obliging us to climb for a short distance to round a steep rock-face before descending the Crête de Saleina, the boulder-pocked and tree-covered lateral morain of a long-retreated glacier. It was only mid-morning and the still valley air was already heating up.

Lower valley of the Drance du Ferret

At the base of the morain, we crossed a broad flat to enter the village of Praz-de-Fort, first through modern holiday chalets, then past the traditional brown wooden valois houses and barns.

The route now crossed the river twice more, passing through several traditional villages and offering great views up and down the valley. The final crossing of the Drance du Ferret brought us to Issert - at 1055m, it was our lowest point on the TMB since leaving Les Houches. As we sipped our coffee on a narrow terrace in Issert, we could look up to our destination 400m above in a hanging valley beneath the bulky peak of Le Catogne - Champex-Lac was waiting.

Wood carving on the Sentier des Champignons

A final stretch on the flat road, followed by a long climb up the Sentier des Champignons, which alternated sharp ascents up spurs with more gentle traverses across the forested slope, brought us to the old terminal morain holding back the waters of the lake above.

On the way we spotted our first (and only) chamois on the slopes above - now that made our day!! Passing one of the many discrete bunkers that the Swiss army has bored into the mountainsides here, we finally emerged into the town of Champex after an hour and a half of perspirational climbing.

Passing the Plan-de-l'Au, the TMB started to head up into the Valley of Six-Fours, dense with larch and fir. Our real climb of the day had just started as we headed steeply up to the bare rock walls of the Clocher d'Arpette above, a narrow torrent plunging down the valley to its right.

A little way along, we crossed a branch of the torrent to follow a rocky path that climbed up a scrubby slope between patches of forest - crossing other branches of the torrent on the way. After the last crossing, a very steep series of zig-zags up a rocky and root-gnarly path beneath the larches brought us out above the tree-line, 400m higher and dripping with perspiration.

The meadows of Bovine

Still, the breeze on the ridge was cool and, as we wandered around the contour line, the views opened out over the pastures and down the broad valley of the Rhone.

View over Martigny and the Rhone Valley from Bovine

Reaching the small buvette at Bovine, we stopped for a well-earned lunch, before one final climb along a track lined with rhododendrons just coming into flower. This took us to our high point of the day at the Collet de Portalo and from here it was all down hill.

The route down sidled around the long valley wall through beautiful fir and larch forest, sometimes steeply, sometimes gently, on a path sometimes even, sometimes rocky and root-covered. As we descended, the road from Martigny in the Rhone Valley below gradually ascended on the far side of the ever narrowing valley. The noise of the traffic grew louder until track and road merged at the Col de la Forclaz.

The initial part of the climb would take us 500m straight up its face in a series of zig-zags beneath the fir and larch forest. Faced with such a climb, there is only one thing to do - find your rhythm matching breath to stride and stick to it - which we did steadily for 50 minutes, passing the early bolters to reach the tree-line across from the impressive rock ramparts of the Croix de Fer, shirts soaked with perspiration.

Thankfully, the hard part of the climb had been in the shade of the forest. From this point, the refuge on the Col de Balme looked deceptively close, but it was still a long haul - traversing the treeless upper slopes of the valley leading up to the pass, slopes covered in rhododendrons just coming into flower. Passing the buildings of the Bergerie des Herbageres, a few more long switchbacks finally brought us to the refuge sitting on the top of the col.

Here we checked into the Auberge de la Boerne, a traditional wooden building that probably breaks every Occupational Health and Safety Regulation known in the way that they pack walkers into dorms. We may well have considered it a charming rabbit warren, had we not ended up sharing an attic with three others, sitting room only and not much more than one cubic metre of space per person! Still, the alternative was to descend further and one does not give up precious metres so easily.