You can also join Trailregistry ( http://trailregistry.com
which also does the same.. Quite a few trails already saved in GPX format.
I'm just beginning to get the hang of how to convert between formats with EasyGPS, with the Magellan Explorist 500LE, and eTrex units I own, (classic, Legend, and Venture-HC) and GPX/XML data formats, but easiest so-far for conversion with the GPSr's and map software on my laptop. (MS Streets & Trips, DeLorme Street Atlas, USA., and Garmin Mapsend Topo)
I'm actually surprised, no-one has put any parts of the Tunxis Trail here in Connecticut.. Time to do a little contribution!
(Side note, Upon posting this, the service is temporarily down.. )
(It should be back up soon.)