The Charles River Path is a 22 mi loop that runs along the Charles River, all the way from Downtown to Newton. The path runs by MIT, BU and Harvard Universities, is fairly flat and runs on both sides of the river with many bridges along the way to cross over. My favorite stretch is ~7.5mi loop- start from the end of Longfellow bridge on the Boston side, run to JFK bridge at Harvard University, cross over and finish back at the Longfellow Bridge. This path is popular amongst runners, from Peter Segal (host of NPR’s Wait Wait), Haruki Murakami (author) to Molly Seidel (Olympic Bronze Medalist for Marathon), many famous personalities have run on this path and written or talked about it. You never know who you might see when you run here!
There are many entry points, the challenge is to find a suitable parking spot! Parking garages in Boston can be expensive but there are plenty of street parking spots that become more easily available on the weekend.