This trial is frequented by cyclists who can appear out of nowhere and are traveling fast. This trail was used as leg 12 of this year's SoCal Ragnar Relay. Details here.
This is a paved, elevated bike path that gradually climbs from the beach to agricultural areas of the San Luis Rey watershed. This is a great trail for racking up mileage without having to worry about traffic. The beach end is near the shops and dining of Oceanside Harbor if you are interested in treating yourself after a run.
East Trailhead: From the I-5 take Hwy 76 East for 6.7 miles to N. Santa Fe Ave. Turn left and the trailhead is on the right. Parking is limited.
West Trailhead: Exit the I-5 at Harbor Dr in Oceanside. Head West and follow the street to the left down a hill. Stay left at the fork. Proceed about 0.5 miles until you reach N. Pacific St. Turn left and go across the bridge and look for parking on your left. One trailhead is immediately across the bridge before you reach the parking. There is another trailhead off of Neptune Way, but there is very little parking there.