I’ve always been slight, all slender muscle wrapped around small bones. I’m an only child raised by a single mother who kept the idea of physical hardiness foreign for much of my life.

I was first exposed to athletic ichor while sitting on a wall outside of Broughton High School, waiting for a friend to finish wrestling practice. Shane limped over later than expected and placed his right hand over one nostril. He blew a stream of hot, red blood from the other, the fluid filling the little inlets of my porous seat, creating a ruby puddle. I was horrified, but Shane looked proud.

For the better part of our lives, Shane and I have tugged one another into experiences both good and bad, watching out for each other in lieu of our less-than-present parents or like siblings we never had. It was Shane who stood uncomfortably next to me when we got caught smoking pot in the high school parking lot. Shane taught me how to lift weights and shocked me with a Taser when I swore I could handle it. I “helped” Shane write his English papers and, years later, convinced Shane’s partner that cats were cool enough to take as pets.

So, when Shane suggested that my husband, Grayson, and I join him in a 15-mile, military-style obstacle course that bills itself as “Probably the Toughest Event on the Planet,” I didn’t say no. The popularity of obstacle races has exploded over the last three years, standing alongside the traditional 26.2-mile run as an ultimate endurance test. While my interests lie more with reality-show marathons, I figured more than a million participants can’t be wrong, right? I began my training: a regimented Internet search to vet each obstacle, punctuated by a pull-up or two, repeated weekly.

When the research revealed the true severity of our challenge, Grayson began to run, perhaps to carry himself as far away from the inevitable as possible. I worked on my upper-body strength, something I’d been meaning to do since dipping out of crew in college. We read about the handful of mud-run deaths and of people breaking bones. The night before the race, we said our goodbyes to our pets, and then to each other. We were afraid.

But we careened through the course, dashing into pits of mud so deep that we had to pull ourselves out by the hooks of our nails. We jumped into vats of water so cold that our muscles immediately threatened deadlock, and we shimmied on our bellies through electrified sludge, ineffectively trying to dodge dangling wires holding 10,000 volts. The surprising part wasn’t that the 15 miles were, at times, excruciating; rather, it was that Grayson led the pack running, and that I scurried up vertical inclines almost effortlessly. For all our fear, we had gotten fit.

Our team finished in a little more than four hours. We were beaten, bruised and smiling from ear to ear. Grayson and I were barely across the finish line before Shanewho carried a 25-pound log through the course to raise an extra $500 for the Wounded Warrior Projectwas congratulating us. As we were exchanging hugs and high fives, I noticed that fresh blood had sprung from either his hands or mine.

I couldn’t tell, but it no longer mattered: We were Tough Mudders now, and so he was a blood brother, indeed.