As far as a trail run goes, this one has a high ratio of hiking, or at least some very careful bouncing along. The trail is either steep or rocky, frequently both, with brief interludes of smooth trail. But, it offers far flung views of the Mont-Blanc massif and a tour through all sorts of curious rock formations. It might be busy, but only with locals.
Starting from the Servoz train station, you'll start accumulating meters as you run up through town to reach the edge of the forest where climbing gets even steeper going up through the trees. The climb only relents briefly as you use a gravel road to cut across a high pasture toward a group of chalets at Ayeres des Perrières. From here you'll begin the adventure towards Passage du Dérochoir which starts as a steep rocky trail, turns to talus hopping following red dots painted on the boulders, and finally is fitted with some metal and rope to help guide you to the saddle.
From Dérochoir you'll find some pleasant running leading to the last uphill switchbacks to Col de la Portette. Keep an eye out for herds of napping ibex in the talus below Pointe de Platé. Through the narrow, grassy pass, you'll drop into an new landscape, a rapid descent to Chalets de Platé, and an arena of karst. When it looks like you're about to run to the edge of the world, you'll drop onto the Egratz, a wall of shale with a well worn and surprisingly runnable series of "S" curves.
Now you've got a bit of work to get back to where you started. Gravel forest track leads to an unfortunate stretch of ascending asphalt to Station de Plaine Joux and Lac Vert. From Lac Vert, you'll connect sections of steep forest trail and stretches of road to drop back to Servoz and a collapse onto a train seat.
Detailed GPX Track
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Start: 45.924442, 6.763265
This is a long climb and a long descent. If you have a car, you can save yourself a handful of kilometers and meters by parking at Station de Plaine Joux (1337m), but the windy drive up doesn't save all that much time.
Pay attention to the GPX track, there are a lot of turns and intersecting trails.
Karst is especially slick on wet days. Most of this route is better in dry weather.