Southeast of Zurich, St. Moritz (pop. 5100) is a real paradise for serious bikers. There are over 75 miles of tracks around the town plus another 200 miles along the valley that were set up for winter nordic skiing.
One of the more popular routes is the Engadin Bike Marathon Trail. It is a 78kmmountain bike route encircling the greater St Mortiz region.
Also, Check out the Val D Vina Trail which is chiseled into the mountain side.
To reach this area from Zurich, head southeast on Route A3 and then south on Route A13 to Chur. Turn south on road to St. Moritz for 70km.
Overall, St. Moritz Engadin Bike Marathon Trail is the 5th most popular mountain bike trail of all 30 mountain biking rides in Switzerland.