When I lived in Del Mar and worked in Poway, I used to take my mountain bike that way. Los penasquitos preserve was my main thoroughfare. It was kind of surreal to do nearly that whole <20 miles almost exclusively on dirt. Cool swath of natural space, but some of the stuff nearer to Poway were the creek goes under the 15 was more interesting and less visited.
There is a myriad of trails in that preserve from straightforward fireroad to tangles of single track.
