I started a loop of about 60+ miles last year, but did not finish due to weather. It might even have been longer, I would have to look at a map again. I started at Lake Morena park and took the PCT north and cut across the inlet to the lake and headed west on Morena Stokes Rd for a mile or so and hit the Espinosa trail south of Corte Madera. We followed that to the Secret Canyon trail in the Pine Creek Wilderness. Take that trail to the trailhead in Pine Valley and walk down Old hwy 80 to Pine Creek Rd. Follow that to Noble Canyon trailhead and take that to Indian Creek trail, follow that up to the East Mesa in Cuyamaca, work north towards the east side of Lake Cuyamaca, there are a couple trails that I can't remember the names of. From there hit the PCT south from Fages Monument and head back to Lake Morena. The only bad spot us Secret Canyon, very overgrown and trash from illegal immigrants. Tom Harrison map is all you should need to piece it together.
You could look at some trips out of McCain Valley down into the desert, I'm sure you could put some 20+ mile trips together out there.