Yup I ride in bethpage park. The best thing to do is start in the parking lot and just ride around. If you follow the power lines they will take you into the brickyard, thats a great place to test your skills. The park itself does not have a marked trail system. You just go in and ride around. If you feel that you are getting lost, stop and listen for the traffic noise from the Seaford Oyster bay expressway, head opposite from the noise and you are heading to the main parking lot.
If you are feeling ambitious you can start in bethpage park, follow the greenbelt north into stillwell woods, ride a loop of stillwell , and south on the greenbelt back to the park.
Bethpage park is not the most difficult trail on the island. It will test your endurance because you can get 22 miles of trail in the park if you loop everything correctly.