Words of mountain biking wisdom from Joel Erickson:
(from email dated October 9, 2001)

There is Mercer County park, which is about five or so miles away.
Follow Alexander Rd. across Rt. 1 to the end (going south) and turn
right. Follow this road a ways. The road bears left at one point, but it
is possible to continue straight past some signs that say "Do Not
Enter." You will find yourself in a gravel parking lot near a pond (or
lake, depending on your point of view). There will be trails to your
left. Mercer isn't too technical, although there are a bunch of logs and
some planks and jumps set up by the local mountain bike and BMX crew.
Mercer is intractable in the mud, but fun in snow and ice.

Another option is Sourlands Nature Preserve (or something). I'm not sure
what the directions out to it are, but you should be able to locate it
on a map. It has a hill and is pretty technical (rocky).

Somewhat further away is Clayton park. It's not very technical but
somewhat rolling.

Tyler tells me that the Princeton Day School on Great Rd. has some
mountain biking behind it.

There is a little park off of the road with the last sprint on the
sprint loop. It's maybe a mile after the sprint point. I'm not sure what
it's called, but if you follow Prospect to Nassau, turn right, left on
River Rd. and then first left and continue for a while, you'll come to
it on your right. I'm not sure if it's really kosher to mountain bike
there, but I often go there for cyclocross workouts. It has logs, but
not much else.

There's also Ringwood State park (where the Army race is). It's
about an hour and a half north. I haven't been there, but it's supposed
to be fairly technical.

If all you want is a workout, there's always the towpath and Institute
Woods.

Joel