THIRTEEN MILES into the Elk Mountain Grand Traverse, a 40-mile backcountry ski race that follows the historic mail route between Crested Butte and Aspen, my teammate and I come stumbling into the first checkpoint. Since midnight, in frigid, wee-hour winter conditions, we've been grinding uphill, following the weak beams of our headlamps. It's now 4:30 a.m. Nick has been patching blisters since mile two, and I've lost most of the feeling in my fingers. Looming before us is a thousand-foot climb to 12,303-foot Star Pass; on the other side, the first whiff of a downhill. Looking like two punch-drunk middleweights, we gorge on crumbly Fig Newtons. When I stick my CamelBak hose in my mouth, the frozen bite valve snaps off in my teeth. I'm having the time of my life. Seriously.

The arrival of winter used to mean a four-month endurance-sports hiatus, but a decadelong infatuation with summertime adventure races has created a demand for equivalent cold-weather tests. These new events—which often involve a combination of skiing, running, snowshoeing, and cycling, both on snow and off—exact the same physical demands as a marathon, with your stamina and skill getting pushed to the limit by harsh conditions. Dozens of winter multisport races now take place around the country, from the Iditasport 100, a sufferfest held each February near Anchorage, Alaska, to New Hampshire's Son of Inferno Pentathlon, held in April at Tuckerman Ravine.

Why submit yourself to such torture? For us, that question was answered when we emerged over Star Pass and were greeted by a mountain vista stretching 50 miles into the dawn: Because these events offer a chance to test your fitness in the kind of snow-laden wonderlands that chairlifts can't reach.

Guidelines

DON'T WORRY. Winter races come in all shapes and sizes. HERE are SOME guidelines to help you FIND THE PERFECT FIT.

YOU ARE: a reasonably fit weekend warrior who's never tried a winter event.
YOU SHOULD: get your feet wet by participating on a team. Both the Mount Taylor Quad and the Son of Inferno (see below) offer team divisions, meaning you have to compete only in a single race leg (a 30-mile bike ride, say, or a 10-mile run). Generally speaking, you'll need to sustain one to two hours of aerobic activity. Prep at least eight weeks prior to the event by gradually increasing your weekend workouts until you're able to comfortably complete the actual race distance.

YOU ARE: a recreational athlete who works out three to four times a week and who has completed at least one half-marathon or triathlon.
YOU SHOULD: try a race solo. This requires sustaining aerobic output for three to six hours. Buttress your endurance beginning two months prior to the event by stacking several race disciplines into one workout (e.g. 45 minutes of snowshoeing, an hour bike ride, a half-hour ski). Slightly increase the duration of these workouts each week. Supplement with three hourlong weekday runs or rides.

YOU ARE: a serious endurance athlete with numerous summer and at least a few winter races under your belt.
YOU SHOULD: tackle the Elk Mountain Grand Traverse or the Iditasport 100. Be prepared for at least eight hours of aerobic output (we'll assume you already have an appropriate endurance-training regimen). You'll also need solid winter backcountry skills, such as identifying signs of hypothermia, being prepared for an emergency bivouac, and knowing how to fuel your body for sustained activity in cold, dark environments.

Events

Test yourself. Take a winter race like one of these.

Iditasport 100
February 8
Sheep Mountain, Alaska

THE RACE: "CAUTION: Weather can be extremely cold!" reads the race Web site. That's your first hint that this is a serious undertaking. Up until the starting gun, you can choose between a bike, skis, or snowshoes to take you 100 miles alongside the 10,000-foot Talkeetna Mountains. Even surrounded by fellow competitors, most first-timers from the Lower 48 will find it the most pristine wilderness environment they've ever experienced. HIGHLIGHT: Finishing. No other winter race in America offers a more formidable challenge. CRUCIAL GEAR CHOICE: Bikers have the advantage. Arm your frame with aggressive, studded treads—try Nokian's Extreme 296 ($112; www.nokian.com). And run your tires soft.THE LOWDOWN: Entry fee $250; 907-345-4505; www.iditasport.com

The Mount Taylor Winter Quadrathlon
February 15
Grants, New Mexico

THE RACE: The "Quad" entails a 13-mile road ride, five-mile run, two-mile ski, and one-mile snowshoe from the town of Grants to the top of Mount Taylor, an 11,301-foot extinct volcano in west-central New Mexico. After a 4,800-foot ascent, and a brief medical check on top, you reverse the entire course and head downhill back to town. HIGHLIGHT: Hopping on your bike for the final leg of the race, a fast 13-mile descent to the finish line. CRUCIAL GEAR CHOICE: Climbing skins are essential for the last mile of the cross-country ski climb, dubbed Heartbreak Hill. THE LOWDOWN: Entry fee: $75 ($55 before Feb. 1); 800-748-2142; www.mttaylorquad.org.

The Elk Mountain Grand Traverse
March 28-30
Crested Butte, Colorado

THE RACE: Starting at midnight from Crested Butte, 100 teams of two ski 39 miles over two 12,000-foot passes and through some of the most rugged mountain terrain in the state en route to the summit of Aspen Mountain. From there, it's a glorious 3,000-foot drop on groomed runs to the finish line. HIGHLIGHT: Besides the guarantee you'll never race in a more beautiful winter setting, the finish line fiesta at Aspen's base area is well worth the 40-mile grind. CRUCIAL GEAR CHOICE: Most participants prefer lightweight cross-country skis with climbing scales and a metal edge (see left). THE LOWDOWN: $100; 970-349-1019; www.elkmountaintraverse.org.

Son of Inferno Pentathlon
April 18-20
Glen, New Hampshire

THE RACE: Run seven winding miles, kayak six miles of Class I and II rapids, and pedal the 18-mile climb to Pinkham Notch at the base of Mount Washington. Okay, now stomp up 4,000 feet to the top of the most storied backcountry ski slope in America: Tuckerman Ravine. Finally, slap on your skis for the 2,000-foot drop to the finish line. HIGHLIGHT: Nine soloists completed last year's inaugural event. Start training now and you could own a course record. CRUCIAL GEAR CHOICE: A helmet, for skiing Tuck's 50-degree headwall. THE LOWDOWN: $100 individual, $500 team; 603-356-0131; www.friendsoftuckerman.org.