I live In East County San Diego (La Mesa) and am having a little bit of difficulty finding decent extended rides.
There's the recently built path along the river out through Santee, but that's fairly short and doable on a raodbike.
I generally do a 20 mile loop out from La Mesa, through the rideable parts of Mission Trails, onto the river path to Santee, that takes you over to Lake Jennings area. There's still a good bit of busy-traffic pavement though.
I've tried driving out east to Anzo Borrego and Borrego Springs. The dirt roads out there have a lot of soft sand that was fairly difficult even on my 29er, kind of fat-bike territory. I go there often to ride the roads or MTB. Seems like there's not much in between.
I'm wondering if there's any networks of fire roads around here.
I've done a lot of MTBing out in Cuyamaca, but that stuff can get pretty dicey on a gravel bike.
I know about the short little sections along the freeways, Sycamore Canyon, Penasquitos Preserve, all surrounded by urban sprawl and relatively short.
Trying to do 40 to 60 mile route if possible with minimal cars. Not complaining though! If we simply don't have it here, so be it, hoping to take a bikepacking dirt road vacation soon and will gladly drive/amtrak to another region if needed.
It's a bit further away but there is a lot of riding up in the Lake Hodges area - the San Dieguito River Trail / Coast to Crest Trail. We've ridden from the Wild Animal Park to Dog Beach in Del Mar almost exclusively on dirt. We've also ridden a big loop around Lake Hodges via Lake Poway / Los Penasquitos Cyn / Lusardi Loop / Del Dios Gorge though that's probably more suited to a mtn bike. There is also some nice riding North of Ramona in the (Big) Black Mtn Area - the Santa Ysabel Truck Trail and Black Cyn Rd.
Do a search for Gravelstoke and/or Gravel Dave on Facebook. Gravelstoke also has a regular website and is putting together some pretty nice rides. you can also search for some of the old Belgian Waffle Ride routes for some ideas.
As a former Escondido resident I'd re-mention the Lake Hodges trails noted above. North County trail advocates have been planning and working on the Coast to Crest trail connects for years which will enable some really long hikes (and hopefully also be bike-legal).
I know it's not San Diego County, but Temecula is full of dirt roads. Look up SPNDX stampede as they've done a few fondo's in Temecula and the roads aren't nearly as chunky as a lot of the stuff you'll ride in San Diego.
Peter Stetina's "FKT San Diego" route is a combination of Hodges and Ramona area. You do have to climb highland valley - which can get sketchy, but it's a good route.
But to answer the op's question - there's very few extended dirt roads in San Diego County. Nothing that will be 30-40 miles of uninterrupted dirt riding. Most of the roads are 2 to 10 miles in length, and then you need to jump on a road to get to another dirt section.
Thanks all for the additional input! Will look into all of these, but if anyone has any route map links that they’ve done to share that would be awesome