Pope John Paul II Park, Boston The Boston Harborwalk Loop is a 2.7-mile easy trail, with an additional 1.4-mile easy loop trailavailable in the park. The paved trail along the Neponset River starts at Pope John Paul II Park and goes north to the boat harbor. The trail surface is paved concrete, smooth, and typically at least 6 feet wide, and there are 4 van-accessible parking spaces near the south end trailheadfor accessibility. This 66-acre state park borders the Neponset River in Dorchesterand features a restored salt marsh area with native trees and shrubs that attract snowy egrets and great blue herons. The area is popular for birding, road biking, and running.