Mont Blanc Marathon Route: Profiles and Difficulty by Distance
Written by Victor Michel — history enthusiast and passionate alpinist based in Chamonix. Published August 6, 2026
The three main distances of the Mont Blanc Marathon — 42 km, 23 km and 90 km — share part of the route but have very different profiles. The 42 km concentrates its technical difficulty on the Aiguillette des Posettes, the 23 km remains the most runnable of the three, the 90 km links nine checkpoints over roughly 6,200 m of elevation gain.
Key takeaways
- 42 km: 2,540 m of elevation gain, technical difficulty concentrated on the Aiguillette des Posettes
- 23 km: 1,680 m of elevation gain, the most runnable of the three formats
- 90 km: roughly 6,200 m of elevation gain, 9 intermediate checkpoints, 24-hour cut-off
- The three routes share part of their course through the Chamonix valley
The 42 km: the technicality of the Aiguillette des Posettes
The historic format of the Mont Blanc Marathon starts on runnable trails before switching, in its second half, to the technical slopes of the Aiguillette des Posettes — the route's main difficulty, a steep sector that contrasts with the rest of the course. The profile then continues toward Vallorcine and then la Flégère before descending back to Chamonix, for a total of 2,540 m of elevation gain over 42 km.
The 23 km: the most runnable format
With 1,680 m of elevation gain and 870 m of descent, the 23 km is the most accessible of the Marathon's three major formats. The route passes through Montroc and then la Flégère before finishing at Planpraz, without a section as technical as the Aiguillette des Posettes on the 42 km — a profile that makes it a good entry point into the longer formats.
The 90 km: nine checkpoints, 24 hours of racing
The Mont Blanc Marathon's ultra-long-distance format links nine intermediate checkpoints — la Flégère, Buet, la Villaz, Emosson, le Châtelard, le Tour, les Bois, Montenvers and the Plan de l'Aiguille refuge — over roughly 6,200 m of elevation gain and 88 km. The maximum cut-off time is set at 24 hours from the start of the last wave.
A shared course, different demands
The three routes share part of the trail network in the Chamonix valley, but diverge on the technical sections and the altitude reached. A runner preparing for the 90 km often recognises portions of the 42 km or the 23 km during training outings, though experience on one does not fully prepare for the specific difficulty of the other.
Sources and resources
Read next
FAQ
What is the most technical part of the Mont Blanc Marathon's 42 km?
The Aiguillette des Posettes, in the second half of the route. It is a steep sector that contrasts with the more runnable trails of the rest of the course, and the main technical difficulty of the 42 km.
What is the elevation gain of the Mont Blanc Marathon's 23 km?
1,680 m of elevation gain and 870 m of descent, on a route that passes through Montroc and then la Flégère before finishing at Planpraz. It is the most runnable of the Marathon's three major formats.
How many checkpoints are there on the 90 km?
Nine: la Flégère, Buet, la Villaz, Emosson, le Châtelard, le Tour, les Bois, Montenvers and the Plan de l'Aiguille refuge, before the finish in Chamonix. The maximum cut-off time is 24 hours.
Do the Mont Blanc Marathon routes share the same course?
Partly. The three formats share part of the Chamonix valley trail network, but diverge on the technical sections specific to each distance — the Aiguillette des Posettes belongs only to the 42 km, for example.
Is the 90 km technically harder than the 42 km?
The difficulty of the 90 km comes mainly from its length and cumulative elevation gain (roughly 6,200 m), rather than a single especially technical sector like the Aiguillette des Posettes on the 42 km. These are two different forms of demand.