The sun is seen through ice formed on a window in Burnsville, Minn., on Wednesday, as a deadly arctic deep freeze enveloped the Midwest with record-breaking temperatures. (Brian Peterson/Star Tribune/AP)
Faye Norby is an ultramarathoner and epidemiologist at the University of Minnesota.

For trail runners, between zero and 15 degrees Fahrenheit is a very comfortable range for winter racing. Those were the temperatures during much of the Tuscobia Winter Ultra competition a month ago, an ultramarathon that traversed 160 miles over some 60 hours in Wisconsin.

But the Arrowhead 135, in Minnesota, was expected to get colder. A lot colder. As I saw the forecast predict highs of minus-20 degrees and lows of minus-35, I got nervous. I grew up in North Dakota; I don’t mind a little chill. But temperatures that low can be dangerous if you don’t know what you’re doing. The margin of error is very small.

I strategized about my gear. I taped up my feet — there were blisters beneath my nails, from running 45 miles through slush during the Tuscobia — and put on two pairs of socks. I laced up my trail running shoes, their toe-boxes wind-proofed by duct tape. I’d bought them one size too big, so I could have room to wiggle (it’s important to be able to stretch your feet).

For me, suiting up for a distance race is all about putting on lots of quick-drying layers, with strategic zippers so I can vent heat. I wound a balaclava around my head and pulled an overwinter mask on top. I also put tape on my cheekbones and across my nose — that way, any frost would build up there instead of sitting directly on my flesh, and the icy edges of my face layers wouldn’t chafe as much. I had a kind of horrifying Darth Vader look, but beneath all of this, I could stay pretty comfortable.

All the runners pull sleds tied to our waists, piled with mandatory equipment, like a sleeping bag rated for minus-20 or colder and a little stove and pot to melt snow for water. With my extra clothes, a small cooler stocked with food and a liquid electrolyte mix and other supplies, my sled weighs about 38 pounds. Some of the competitors who are racing to win — let’s be honest, it’s almost always the men — obsess about shaving down the weight, but I’m not one to skimp. When you try to get by on very little, sometimes you set records, and sometimes you’re forced to drop out. Anyway, the sled doesn’t feel as heavy as you’d think. We’re moving on a snowmobile path, so the snow is well-packed, and it slides along nicely when you’re not going uphill.

What’s amazing about winter ultramarathons is you get to eat everything you couldn’t in normal life. I made up a jumbo trail mix of junk that stays edible when frozen — animal crackers, chocolate-covered pretzels, chocolate sandwich cookies, Ritz cracker sandwiches — which I divide into zip-top bags and then put in an animal treat dispenser, for efficient consumption. Some people thaw food in their pockets, against their bodies, but I don’t want to have to open my zippers too much. Zippers can freeze, and then you’re stuck with open clothes.

When I started at 7 a.m. Monday in International Falls, along with 64 other competitors on foot (others are skiing, cycling or kick-sledding), I thought finishing in about 50 hours would be doable. When I’m actually out there on the trail, though, I adjust my expectations. First and foremost, I just wanted to finish.

I have to make smart decisions at every turn, even if they are just little things. I would stop only in spots that were out of the wind, or in a little patch of sunshine. Even when I paused to get food or hand warmers to keep my mittens from freezing solid, I had to keep myself warm — I also had to keep myself awake. As the hours and miles wore on, after each little 40-second stop, I would make myself stand and run. To keep my mind occupied, I listened to audiobooks: I’d loaded a few titles onto my iPod just for this purpose, like “Murder on the Orient Express” and Blair Braverman’s memoir about her adventures in the north (I thought all her writing about sled dogs would feel inspirational).

Time and energy were precious. I would think to myself, “It will take 15 seconds to open this packet of food, and I don’t want to do that, but I have to.” At a certain point, I had been up for two nights, and the thought of eating made me tired. It got hard to even make myself drink water from my hydro flask’s hose.

The most sleep I got was a 45-minute lie-down at the second of three checkpoints, at a cabin — but I’m not certain I actually did sleep. There was a lot of noise from people coming and going; this was the spot where many people decided to end their races, and they waited for rides back to the start. Some experienced runners recommend lying down your sled at regular intervals for quick five-minute naps. Even if you don’t really sleep, shutting your eyes is a good reset for your brain. But even with my sleeping bag, I was worried about exposure to the wind — it didn’t feel safe to stop.

Even though I’m pretty even-keeled for an ultramarathoner, I definitely faced some hills when I thought, “This really sucks.” I was so exhausted the slopes felt ridiculous to me. They were steep, and that section was never-ending. It just kept going, one hill after another. Though I didn’t seem to hallucinate, the way some distance runners say they do, I did start to see some strange shapes. Navigating by the light of my headlamp through the woods, out of the corner of my eye I’d see animals or faces in the piles of snow sitting on branches.

The finish line at Tower was very quiet at around 7 a.m. Wednesday. The temperature was minus-33 degrees — minus-59 with wind chill. I got to see the most beautiful sunrise. When I crossed after 48 hours and 34 minutes, I was the first woman to finish. It felt amazing to get to the lodge at the end of the trail, leave my sled in some hallway and take off all those layers — including what felt like pounds upon pounds of ice. I showered and then slept. When I woke up, the volunteers who administer the race had ordered pizza. Only 36 percent of the 146 total participants ended up completing the race this year, but there wasn’t a lot of fanfare. For me, that’s part of the appeal. I’d run a few marathons before I got into trail running seven years ago and quickly found its style suited me better — more laid-back. Trails are easier on your body than roads, and the people are friendly. After all we’d endured during the Arrowhead, everything felt very calm.

My next race, in a few weeks, is 100 miles — in Canada.

As told to The Washington Post editor Sophia Nguyen.