Les 21 km de l'Yvette

Les 21 km de l'Yvette is a nature run organised by the town council of Bures-sur-Yvette, in the Essonne department just south-west of Paris, at the gateway to the Chevreuse valley. First held in 2025, the race quickly found its audience, with more than 350 finishers at the very first edition. It takes place on a Sunday morning in late March and, over a half-marathon distance, takes runners along the paths linking the towns of the Yvette valley, between Gif-sur-Yvette, Orsay and Chevreuse.

The race starts at 10:00 am inside the Louis Chabrat stadium, where the finish line is also located. The 21 km course mixes tarmac roads, forest trails and dirt paths, winding along the wooded slopes on either side of the river. The profile is rolling, hilly enough that the organisers consider it unsuitable for wheelchair athletes, but without the long climbs of a mountain trail. It is ideal for road runners wanting to try off-road running, or for trail runners looking for an up-tempo outing in early spring. Every kilometre is marked and the route is flagged with tape. The time limit is three hours, and the fastest runners finish in around an hour and a quarter.

The choice of start is no coincidence: the Louis Chabrat stadium is also the hub of the town's trail station, a network of seven waymarked routes of varying difficulty, plus technique workshops, spreading out through the surrounding woods and valleys as far as the Mérantaise valley. Local runners know these paths well, and the race offers a morning-long sample of them in a very green setting just a few kilometres from the Saclay plateau.

The race is open to runners aged 18 and over who hold an FFA licence or a valid Parcours Prévention Santé, the French federation's online health certificate. The field is capped at 500 and registration is online only, with no entries on the day. Three aid stations offering water and dried fruit are set up at around 6, 10 and 15 km, with a final refreshment area at the finish. A cyclist sweeps the course and medical cover is provided along the route.

Bibs are handed out on race morning from 8:00 am at the stadium gym on Rue du Docteur Collé, on presentation of photo ID, and a bag drop is available. Parking is not allowed inside the sports complex, but Bures-sur-Yvette station on RER line B is only a few minutes' walk away. The organisers recommend carrying a phone, a foil blanket, a warm waterproof layer, gloves and a hat, as late-March weather can be unpredictable.