There's the Seven Sisters race done near Holyoke, MA. One of the park rangers told me they get a decent number of injuries from that and keep a bunch of ambulances and EMTs ready at the end. I think its about 10 miles one way with parking at either end, if you take the longest trail route through there, and with the up and down and the scree slopes it can be a grind. Nice views. The route can be extended to longer by heading eastward on the Robert Frost trail; I doubt that additional stretch has any great difficulty though. The Seven Sisters can take longer than you might expect, I think it cut my typical trail speed in at least half. Not sure if it warrants a "top 10" list, but it's rated a difficult trail and I think that is accurate.