A great route that features both gravel roads and some singletrack trails; runners will enjoy making their way through the Parc Natural ae la Serra de Collserola.
Take a taxi from Barcelona city to the Tibidabo Monastary/amusement park. Approximately 1 km down from Tibidabo there's a forest on the left hand side, where the Parc de Collserola paths start. It's down-down-down, following the natural trail trough the whole park. There are some rocky, singletrack parts in the start, but later the trail is mostly just dirt roads. There's always lots of people in the area to ask for directions.
Once you get to Sant Cugat, take a taxi back to Barcelona.
Flora & Fauna
There's some warning signs about wild hogs in the end, but I haven't seen any.