The Orr Springs 600K is a 380-mile clockwise loop from San Francisco to Ukiah, in Mendocino County, and back. It explores remote roads—some of them unpaved—in the Mendocino Range and the Mayacama Mountains, and accumulates over 29,000 feet of climbing. This course challenges even strong riders, but it rewards with fantastic scenery and uncommon isolation. It is a colossal experience, possibly closer to a 1200Km grande randonnée than to a traditional 600Km brevet.

The GPS claims 383 miles and 29,029 feet of climbing, but the numbers are reductive. The Orr Springs 600 is much more: three-hour stretches without a car in sight; ripe blackberries by the fistful; long dirt climbs in the shadow of Douglas firs; the Milky Way aglow over a pitch-black landscape; dawn over the Mayacamas; the feeling of coming home after having been far, far away.

We start in darkness, at 4am, to make the most of the first day of riding and improve the chances of overnight rest in Ukiah. From San Francisco to Tomales we retrace the return leg of the Old Cazadero 300. The road feels very different in this direction and so early in the day: the air is cold, the ocean dark beneath the cliffs. We ride twenty or thirty miles between car sightings. The only downside is that Point Reyes Station (mile 35) is still asleep at this hour—no pastries to be had from Bovine Bakery.

The wide-open views and rolling farmland of coastal Marin end near Freestone. We enter redwood forest and climb the gentle grades of Bohemian Highway to the first control in Occidental (mile 67). It's wise to keep this stop short—the legs are still fresh and the climbing has hardly started.

Redwoods shade a fast descent to the Russian River, and before long we're in the hamlet of Cazadero, a crossroads of the SFR adventure series brevets. For the next 25 miles we retrace one of the most scenic sections of the King Ridge 400. King Ridge Road follows Austin Creek at first, then rises steeply via a series of tight turns. The redwoods of the valley floor give way to stands of oak and laurel, which in turn give way to open meadows as we climb. The road clings to a ridge barely wider than the pavement itself, and sublime views open up in every direction. To the west, ocean meets sky in a pale blue haze. To the north, serrated ridges recede into the distance, foreshadowing the road to Ukiah.

A steep, sometimes hair-raising descent on Skaggs Springs Road drops us to within a stone's throw of the surf at Stewarts Point. Ten miles further north we cross the Gualala River and enter Mendocino County. The Gualala control (mile 120) makes a great lunch spot.

Now comes a highlight of the course: Fish Rock Road, a rarely used and largely unpaved connector between the Mendocino coast and Anderson Valley wine country. Steep asphalt climbs from sea level to 1300 feet before dropping back down to 800. Then the pavement ends, and it's 3.8 miles of loose gravel at 9% average grade, with ramps in excess of 15%, to the top of the ridge. The dirt continues for five more miles before reverting to rough pavement. Much of the road is wooded, but the occasional lookout reveals how far we've climbed. A corkscrew descent with a posted 5mph speed limit makes for a memorable photo. Eventually we emerge onto Highway 128, dazed by the sudden presence of cars and armed with a renewed appreciation for the capabilities of the bicycle.

Boonville (mile 160) is a fine choice for an early dinner and also a good place to stock up on food for the road. There are no reliable services until Ukiah, 56 miles (and approximately 6000 feet of climbing) away. After a brisk run down the Anderson Valley, we turn right onto Flynn Creek Road, where lush forest, silken pavement, and low traffic create perfect cycling conditions.

For most of us, night falls somewhere on Orr Springs Road between Comptche and Ukiah. The Orr Hot Springs were part of the inspiration for this route, but sadly, their pricing and reservation policies make them unsuitable for a brevet control location. However, the road is remarkable in its own right. It consists of two climbs totaling almost 4000 feet of elevation gain, separated by a descent deep into the South Fork Big River valley, where the hot springs lie. In many places the road is barely wider than a pickup truck, and the pavement is marginal at best. Darkness in these parts deprives us of grand vistas, but the night sky makes up for that loss: there is no light pollution here, and the stars seem close enough to touch. Consider stopping, turning off all lights, and admiring the universe for a minute.

Ukiah's brightly lit main drag makes a jarring contrast to these mountain roads, but the local Safeway (mile 216) is a welcome sight after so many hours on the bike. There are also several motels nearby; it's not a bad idea to get some rest here.

To achieve the time limits of upcoming controls, it is advisable to leave Ukiah before dawn. River Road at this time is deserted, and the flat miles pass quickly. To the east, lightening sky reveals the sharp profile of the Mayacama Mountains. Near Hopland we turn left, directly into the hills. Old Toll Road is 16 miles of well-packed dirt and gentle grades. Oaks festooned in Spanish moss arc over the road, and the glow of sunrise bathes rugged hills of chaparral. There are no cars.

The ridge now behind us, we ascend from the flats of Lake County to Cobb Mountain, the highest summit in the Mayacamas. The paved road up the north side is long but never steep. The top of the climb is the highest point of this ride, and it provides the potential energy for a fabulous high-speed descent into Middletown (mile 276). Breakfast at Hardester's feels well-earned.

Two miles out of town comes the final major challenge of the ride. Western Mines Road is a twisty 3-mile strip of loose dirt that climbs 1500 feet over the eastern shoulder of Mount St Helena. It's not the longest climb of our ride, but it feels like the toughest. There's no shame in walking some of the steep turns. Pavement, however rough, resumes at the summit, and in short order we emerge from tree cover to admire sweeping views of Napa wine country and the western side of Mount St Helena. On a clear day we might see Mount Tam.

The longest continuous descent of our ride takes us into the Napa Valley, and from there it's less than 30 miles, mostly on flat or rolling roads, to lunch and the penultimate control in Sebastopol (mile 319). This is Gravenstein apple country—although many of the orchards, sadly, are being replaced by more profitable vineyards—and the smell of hay and apples fills the air on Pleasant Hill and Bloomfield Road.

The final 50 miles to San Francisco are familiar territory for local riders. Marin welcomes us with its pastoral scenery and—quite possibly, in the afternoon—a solid tail wind. The road is rarely flat, but the long grinding climbs of Sonoma and Mendocino are behind us. The biggest remaining obstacle is Wilson Hill: take a moment to look back from its summit to the distinctive profile of Mount St Helena on the horizon, and consider how far we've traveled. It's not quite all downhill to San Francisco from here, but it almost could be.

Some perspectives on the Orr Springs 600

This is an uncommon 600K, and most cyclists will be close to the brevet time limits for much of the ride. Here are some ways to think about this ride if you are a randonneur or endurance cyclist:

• If a 1200K event is too much or simply won't fit into your schedule, the OS600 is a worthy stand-alone season goal. We recommend preparing for it by riding a full “Super Randonneur” series of 200-600K brevets.
• If you are riding a 1200K later in the summer, the OS600 makes for pretty good training—especially if you're planning on a hilly 1200.
• If you simply want to absorb a great variety of California scenery in one weekend, this ride is just the ticket.