This great loop for running or hiking goes from Valldoreix to the small village of La Rierada via trails and dirt roads. The terrain is mostly woods and shrubby hills with a fair amount of shade.
The exact distance is 11.5 Km, and the accumulated climb is probably around 300 meters. To access the trail head by car, head to Carrer Sant Juan de la Creu, 22. This is only about 200 meters from the Valldoreix FGC train station, so public transportation is a viable option as well.
The turn by turn direction and the map can be obtained on my Wikiloc page
This loop is also a great reference line to depart from and explore several interesting areas of the southern part of the park, such as El Ermita de la Salut or Santa Creu d'Olorda