was in Garfagnana last autumn, it's stunning.
we took mountain bikes, and found great looking trails that were extremely overgrown.
(most of the people who lived in the villages have moved down into the towns - the paths between the villages are still there, but noones used them for 50years)
we had a look on the internet, and found loads of 'mtb' rides (gpx stuff) that consisted of 45k of road riding, for a 3k gravel road descent.
i'm sure there's plenty of ace mountain biking, but you'll struggle to find it - it's not like the alps where you can find a weeks worth of riding just by taking a chairlift, heading left/right, and looking for singletrack.
we should have take our road bikes.