In Corsica, they have the GR 20, reputed to be the most difficult of the French Grande Randonnée (GR) trails. In the Île-de-France region, we're a long way from rivalling the circuit's 200 kilometers and 11,000 meters of ascent, yet it's here, in the forest of Fontainebleau, that you'll find one of the region's most demanding hikes.
