Lots of options. Running along either side of the Charles is great -- you can do up to a 20 or so mile loop if you make it all the way to Watertown. The Emerald Necklace takes you to Jamaica Pond, and these neighborhoods are a bit more affordable than many others. One final suggestion - I used to live near Alewife, near the Cambridge/Arlington line. It provided great access to the Minuteman Bikeway, which was great for running and biking. See included link.