one of the unfortunate things here in North County, if its a really nice track, it is more then likely locked down. I have had good luck with the Mira Costa College track in Oceanside off College near the 78. Stop by the campus police station and tell them you want to run on the track and they will give you a free parking pass. Track is usually empty and is open 8am - 5pm typically.
Cal State San Marcos has a decent track and I have never had a problem running on it, but don't park on campus, they will ticket you. I usually park at the shopping center across from the track and walk over.
If you are looking for a group interval workout, I host an open workout every Tuesday night on the rail trail in Carlsbad. Intervals are an out and back for each one and the paved trail is marked every 100m. We typically have 20 - 30 people show up and split the everyone into three groups based on speed, sub 6 minute pace group, 6 - 7:20 pace group and 7:30+ pace group that includes up to 9:30 pace runners.
If you want to join us just exit the 5 at Carlsbad Village and head west. Right before the RR tracks turn left into the Fish House Vera Cruz parking lot and go to the back. The trail starts there and we meet about 6:15, start the intervals about 6:30pm.
Mike Plumb, TriPower MultiSports
Professional Running, Cycling and Multisport Coaching, F.I.S.T. Certified
http://www.tripower.org