Road-biking doesn't get any better than on San Diego County's remote mountain and desert rural highways -- especially those beyond Palomar and Julian, where traffic thins considerably and the roads themselves are often accompanied by generously wide shoulders.
The bicycle ride down the whole length of County Highway S-2 is one of the easiest, most carefree, and exhilarating experiences you can have. The route, when followed from Highway 79 near Warner Springs to Ocotillo at Interstate 8, drops 2400 feet in 64 miles. There are a couple of nontrivial climbs, to be sure -- but mostly you'll descend in a leisurely stair-step fashion, just as the elevation profile printed here shows. The wind, typically from the west or northwest, will likely be in your favor.
For such a long, one-way, downhill bike trip, you'll want to arrange for someone to drop you off at the start and pick you up at the end. Ideally, you can recruit a driver with a "sag wagon" who can rendezvous with you at various spots along the way and potentially rescue you (or your companions) if you run out of steam along the way.
Highway S-2's alignment, which bridges a gap between the hot desert floor and the cooler mountains, was an important link in the mid-19th-century Southern Emigrant Trail, an all-season route into coastal California. In just four or five hours of pedaling and coasting, you'll travel (on pavement and in the opposite direction, of course) the final dreaded passage faced by ragtag settlers on their way to Los Angeles and points north.
The section of S-2 5 to 15 miles into the ride (roughly from Teofulio Summit to Scissors Crossing) was partially singed by summer's mammoth Pines fire. You'll see where firefighters succeeded and failed in their efforts to stop the fire from jumping east across the highway.
Only a few outposts of civilization punctuate the lonely S-2 route: a tiny store at San Felipe, just past Teofulio Summit; Shelter Valley and its store, past Scissors Crossing (Highway 78 crossing); the Butterfield Ranch resort, just before the frightening drop of Campbell Grade; the Vallecito Stage Station; and Agua Caliente Springs County Park, where you can stop for a timely and welcome soak. Beyond Agua Caliente, you face 26 waterless miles of open road. Keep your water bottle filled. Even in winter the high temperature out there can reach into the 80s.