I live in Merrimack and frequent the trails at horse hill. Most of the trails consist of a mix of double track and some single track. Good for about 1.5 hours worth of riding if you do loop trail, and ledges. There is some newer single track that has just been added but is not posted or on the map yet. The "new trail" is fantastic single track. Not sure I would trek up from Boston to Merrimack for just Horse hill but if you did horse hill and then hit FOMBA in Manchester that would be worth a trip.