Bedford Narrow Gauge Rail Trail. (The Friends of Bedford Depot Park publish a very nice, detailed guide to the trail.)
Not many people on either path, so it was really lovely. Lots of leaves, and I took care not to go slip-sliding away, but I never really lost my footing at any point.
The real surprise was the second path, which picks up very near the Bedford end of the Minuteman path, across Loomis Street. It even has a bit of the old track at the start. The surface starts fully paved, then it turns to packed gravel, much like many of the Sustrans routes in the UK. Again, lots of leaves, but the surface was generally very good, allowing me to truck along at 17mph for most of its length.
I turned around at Fawn Lake (which warrants further exploration another time), which turned out to be precisely 10 miles from home.
Old Thorny performed exceptionally as usual, reminding me that it really is worthy of the odd off-road adventure.