This ride is listed in "Mountain Biking the White Mountains West"...
An older guidebook from Falcon Guides by J. Richard Dornan. I got my copy at Ski Fanatics in Campton, but I am pretty sure you can find it at EMS and other stores as well. The book is about 10 years old, but I have found the descriptions accurate in the main.
I haven't done the Brown Ash Swamp ride, but this book lists it as 6.3 miles starting at Tripoli Road and connecting into Dickey Notch Trail on the other end. The guide lists it as moderate aerobic difficulty, with variable techinical difficult from "pretty easy" on three miles of jeep trails to "very challenging" on 2 miles of tight singletrack (paraphrasing here).
They suggest linking this ride with several others in the area for a full day.