Here's a nice over-and-back route that's challenging but keeps you away from the heavy traffic areas.

1. Get to Back Bay Dr. from where you're starting
2. Follow the other riders, onto a sidewalk along Eastbluff and then Jamboree. Merge onto the San Diego Creek Trail (MUP).
3. Follow the San Diego Creek Trail northwest.
4. Temporary exit at Jeffrey. Ride south along Jeffrey for a mile or so. The MUP will continue on the left side of the road, on a bridge over the 405 to Quail Hill trail (MUP).
5. Follow Quail Hill Trail east to Shady Canyon trail (MUP).
6. Follow Shady Canyon trail south to Bonita Canyon.
7. Follow Bonita Canyon Dr. west to San Miguel (goes through a small shopping center with a Pavilions).
8. Follow San Miguel south to Spyglass Hill (residential area)
9. Follow Spyglass Hill south to San Joaquin Hills (pretty steep climb here through residential)
10. Climb San Joaquin Hills east to Newport Coast (sidewalk OK)
11. Descend Newport coast south toward PCH
12. Take a detour, turning right and descending down Pelican Hill Rd.
13. At the bottom intersection, turn left back onto Newport Coast (sidewalk OK) and climb until you reach Vista Ridge
14. Turn right on Vista Ridge and climb to the summit (this one's steep, over 20% in some areas)
15. Descend down Ridge Park and head back to San Joaquin Hills
16. Backtrack to your car

Should be a bit over 40 miles with over 4000 ft of climbing. Map it on ridewithgps... my directions may not be totally accurate.