The first law of adventure planning – and I say this with confidence, having recently crashed a mountain bike at high speed in the Hawaiian jungle – is that it's not so much where you go as how you decide to get around once you get there. I'd been to Hawaii before, but always to surf, hike, and sightsee, and I'd always rented a car. This time, I thought it would be interesting to see the Big Island – which, at 4,000 square miles, is bigger than all the other Hawaiian islands combined – via mountain bike. My buddy Seamus, who happens to be a beast on the trails, offered to tag along.

I hit the Web and started doing some research. I probably should have worried when I couldn't find a single mention of Big Island mountain-bike touring – no outfitters, no itineraries, nobody even claiming to have done it. Instead, I grew more determined. I spent days playing around with Google Maps, sketching out a five-day figure-eight trek through the island's largely unpopulated backcountry, starting and ending in Hilo, on the island's less-touristed east coast. It looked like the perfect way to experience a Hawaii that few others see.

Day 1: Hilo to Waimea

The greatest mountain-bike ride in the Hawaiian islands is a dirt-and-gravel jeep track called Mana Road. It runs 43 miles around Mauna Kea, one of five Big Island volcanoes, which towers nearly 14,000 feet above sea level. A taxi shuttles us from Hilo to the starting point, but it feels less like a car trip than an elevator ascending to the heavens. He lets us off at 6,500 feet in the windswept middle of nowhere. We saddle up and begin a six-hour journey through shape-shifting terrain. Lava-field deserts give way to parched Mediterranean-like grasslands; California-style oak chaparral bleeds into lush tropical pasture. With Mauna Kea looming overhead, we labor up brutal hills and bomb down what I consider terrifying technical descents. Seamus dismisses them as "child's play."

Wild turkeys wander past the sun-bleached bones of a feral bull, apparently shot and butchered on-site by poachers. Families of wild pigs march single-file in the distance; a father and son pass us in a pickup, intent upon hunting those beasts with dog and knife. The real highlight is the final 10 miles: a nonstop 4,000-foot descent on red dirt through pasture so lush and neon green and speckled with so many yellow flowers that I keep expecting to see unicorns. It drops us right into Waimea, a quaint and tidy town 2,700 feet above sea level, where a hotel room awaits. We feast at a little place called Village Burger, which serves pasture-raised Hawaiian-beef patties on tender white buns with wasabi mayo and a side of parmesan fries.

Day 2: Waimea to Hapuna Beach

It's one thing to experience a fierce island rainstorm in the safety of your rental car. It's quite another to experience it while grinding out a 2,000-vertical-foot climb on a bike. We are on the Kohala Mountain Road heading north from Waimea toward Hawi, a village on the island's northernmost tip, when massive black clouds gather. They burst open, firehosing us with frigid torrents so relentless we can barely see the asphalt even as we rocket downhill at 35 miles per hour. The storm pounds for 20 minutes, ending just as we detour onto remote Pu'uhue Road, which winds through private ranchland down toward the coast.

The warm sun dries our clothes as our bikes fairly float over the deep, pillowy grass that covers the trail. Bands of horses gallop alongside us, their manes flowing in the wind. But the sunshine turns punishing as we merge onto the shadeless shoulder of Route 270, an 18-mile stretch of paved highway, toward the night's destination of Hapuna Beach. Part of the 112-mile bike course for the famed Ironman Hawaii, the road takes us through searing-hot lava fields and past ancient Hawaiian temples. We see humpback whales leaping out of the sea. But I run out of water, and with the sun bearing down so hard that I feel on the verge of sunstroke, a convenience store near Kawaihae Harbor looms like an oasis in the Sahara. Just across the road, at Da Fish House, we devour takeout ahi and octopus poke. Then it's a quick pedal to our hotel, perched above the palm trees and white sands of legendary Hapuna Beach, where a body-surfing session washes away all the grime in time for cocktails on the veranda.

Days 3 & 4: Honokaa to Hilo

We spend our third morning lying around Hapuna Beach before passing back through Waimea toward Honokaa, a little town northwest of Hilo. On the way, we stop at the Mountain Road Cycles in Waimea to get directions to a rocky, slippery, no-cars descent called Mud Lane, which takes us through yet another severe rainstorm and right up to that night's bed-and-breakfast, near the famously lush and gorgeous Waipio Valley Lookout, the setting for a tropical paradise in Kevin Costner's Waterworld. The following morning, we drop by Stay True Cyclery. Owner Colin Bivens volunteers to toss our bikes into his pickup and drive us 3,000 feet up a mountainside to sample the hyper-technical singletracks of Kalopa Native Forest State Park. I spend two hours gripping my handlebars, trying not to get killed, as Seamus dances his bike down what look to me like dry waterfalls. We ride back into Honokaa and eat hot soup at sidewalk tables outside Café Il Mondo. A quick phone call summons the same van service that took us up Mauna Kea on day one, and before we know it we're back in Hilo, checking into the same hotel we stayed in the night we arrived.

Day 5: Hilo

Chris Seymour of the Hilo Bike Hub extends the Big Island hospitality even further. Mistaking me for an experienced mountain biker, he takes the day off, leads us up a series of country roads outside Hilo, and guides us through a heart-thumping circuit of singletrack called Kulani Trails, littered with hardcore obstacles. Seamus kills it. But apparently I have a lot to learn about charging a bike over slick stones the size of watermelons and even more about bunny-hopping slimy tree roots – a feat I somehow manage with my heart slamming and lungs bursting. It was bound to happen: I encounter an obstacle I cannot clear. And that's how I find myself airborne, flying over the handlebars to smack facedown in the Big Island mud.

But that's also how I limp back through the Hilo airport in precisely the state I crave at the end of a great adventure: battered, bruised, and worn-out just enough to guarantee a blissful night's sleep on the red-eye home.

A Side Trip to a Spectacular Beach

The Waipio Valley, located on Hawaii's northern Hamakua Coast, is one of the world's great tropical sanctuaries, a lush green canyon fed by multiple waterfalls that cascade down 2,000-foot-high cliffs. The broad Waipi'o River winds through taro farms along the fertile valley floor (Waipi'o means "curved water" in Hawaiian), where Hawaiian royalty lived for centuries. Now local surfers dominate the thumping waves at Waipio's black-sand beach, while foot trails lead through 18 miles of unspoiled tropical cliffs and beaches beyond.

Getting there is an adventure in itself. Waipi'o Road, one of the steepest roads in the U.S., gains 800 vertical feet in .6 miles with an average grade of 25 percent. It's so steep, in fact, that locals advise taking off your seatbelt and opening the car door before you start – so you can bail out if the brakes fail.

More Information: United Airlines flies once daily from LAX to Hilo. Kona is served by daily flights from the West Coast. Rent a Specialized Rock Hopper 29er for $35 to $45/day at Bike Works. For supplies, repairs, and advice, hit Hilo Bike Hub. The Hilo Seaside Hotel is simple, clean, and near the airport ($105/night); the Jacaranda Inn offers a peaceful country-plantation vibe in Waimea ($159/night); Hapuna Beach Prince Hotel is a beachfront luxury resort ($215/night); Waipio Wayside B&B in Honokaa offers immaculate rooms in a rain-forest home ($140/night).