PLACES TO RUN
New York
* Waterfront Route: 8 miles
Start at West St. and 11th St., follow the bike route down to Stuyvesant (1.5 miles); turn right and follow the coastline down to Battery Park;  cut through Battery Park to reach the Staten Island Ferry Station, then follow the painted fish (to the right of FDR drive) to the South Street (3.5 miles); stay between the road and the chickenwire fence  until you reach FDR park (4.5 miles); turn left and stay next to the East River until you get to the track (6.5 miles); turn after the track and cross the highway using the overpass; run crosstown back to where you started.

* Central Park: 6.1 miles (outer loop); 1.5 miles (reservoir track)
These are pretty self explanatory.  Watch out for the big hill going counterclockwise at the northern end of the park. The reservoir is dirt and has meter markings; the outer loop is paved.
     
Boston
* The Charles River
Dirt and paved paths on both sides of the Charles River for most of its length.

* Fresh Pond: 2.5 miles circumference, 1 mile from Harvard Yard
Follow Garden St. away from Harvard Yard until you get to Concord Ave, which will take you directly to a nasty intersection on the opposite side of which is Fresh Pond.  Paved with dirt paths alongside.  Watch out for poison ivy.

* The Fells
Get lost!

Oxford
* Thames River Path: 8 miles roundtrip to Godstow and back
Pick it up on the far corner of St. Aldate's where it crosses the Isis. Stay on the far side of the river past the locks; at the next bridge, cross over the river and take a left on the first street (detour since the tow path is closed down at the moment); there are signs to get back to the tow path. Mostly dirt with a few paved stretches.

* Oxford Canal
Not as picturesque as the Thames River Path.

* University Parks
If you cross the Cherwell at the really curved bridge, there are a bunch of paths through the hedgerows to explore.