Map My Run is a really neat site that makes use of Google Maps. Click around to set points along your run. The site calculates the distance for you. Useful for other things too.
Some running distances around the river, starting and ending at home.
| Run | Distance (Miles) |
Increase over Base Run |
|---|---|---|
| Short: B.U. Bridge, Mass. Ave. Bridge | 4.0 | - |
| Base: River Bridge, Mass Ave Bridge | 4.8 | - |
| Extending the Base Run to the West | ||
| Western Bridge | 5.4 | 0.6 |
| Foot Bridge | 6.0 | 1.2 |
| Larz Anderson Bridge | 6.5 | 1.7 |
| Elliot Bridge | 7.9 | 3.1 |
| Extending the Base Run to the East | ||
| Longfellow Bridge | 6.8 | 2.0 |
| Charles River Dam | 7.9 | 3.1 |

Of course, one could always just have googled to see if anyone else had done these… This one goes further west and is organized better.
http://www.marathonsports.com/Running/Images/CharlesRiverMap.pdf includes a couple of my other favorite running loops (as well as some slightly annoying advertising for Marathon, which is a really good running store): Fresh Pond and Fenway/Jamaica Pond.
Note to self:
From home, to the river, and back is about 0.9 miles.