New London is about 10 miles or 15 minutes away from Mystic and about 25 miles or 35 minutes from Preston, according to Mapquest. I've never been to the Mystic Seaport festival, but the town itself is worth visiting, even just walking down Main Street. Strawberry Park in Preston is a beautiful campground with resort-like facilities and an incredible bluegrass lineup for this festival.

Both are definitely close enough.