This is a 2.5 mile trail that takes you past some of the most historic buildings and sites in Boston. It's an easy walk. There are sites you can down load maps and info.
Tip: from Faneuil Hall there are free National Park Service tours of the south portion of the Trail, and also free Park Service tours of the Bunker Hill area that leave from the Constitution. Definitely a nice way to learn a little more about the places you are seeing.