There are a few trails located in Puerto Rico's Guanica State Forest, which is located about 30 minutes west of Ponce. You can also bike in Boqueron Bird Sanctuary, which is near Cabo Rojo, a little further west (obviously, you will need a car for either of these options). Can't speak to the accuracy of this site, but there is more info and trail descriptions at this site: http://www.mtb-pr.com/trails_rutas1.htm. You could also try contact them if you need further info.