Running is essentially a solitary act. We may train with others, run every day next to the same group of people, and feel more at home with runners than anybody else, but there’s no one there at mile 20 of a marathon unless you count the voice in your head. That was my experience until yesterday’s New York City Marathon, anyway. I ran all 26.2 miles from Staten Island to Central Park with another racer—often attached to her by a tether. It was one of the most amazing experiences of my life.

In late September, my friend Michael Oliva emailed from Colorado to ask if I would be interested in being a guide for a blind runner in the marathon. She would be flying in a few days before the marathon and we would meet for the first time on Friday, but Mike assured me I would pick it up quickly. I wasn’t so sure, but I agreed to participate, flattered by the offer and intrigued by the challenge.

Amelia Dickerson lost her vision as a teenager and runs with the Colorado chapter of Achilles International, an organization devoted to enabling people with disabilities to participate in mainstream athletics. In August, she set the national blind 5,000 meter record. I was nervous about committing to something that was so unknown to me. But I set about trying to figure out what it would be like and what I needed to know. I read about Amelia’s running and the tether she used with her guide. I asked Jamie Kyei-Frimpong, one of my steady running partners and good friends, to join us, and I read about the specific challenges of guiding in the NYC Marathon.

Perhaps most importantly, I got more serious about my training. This would be Amelia’s first marathon, but she had set an ambitious goal time of 3:30. This is about 30 minutes slower than my typical goal, but I had run two very bad marathons in the last year and had let that disappointment sideline my training. When Mike contacted me at the end of September, there were only five weeks until race day. I hadn’t been marathon training on my own, but I had been coaching a marathon-training program so my fitness wasn’t terrible. Still, I focused on getting in some quality workouts and increased my weekly mileage to ensure I could handle it.

We met at the marathon expo on Friday afternoon, collected our bibs, and received some basic instructions for Sunday morning. Achilles Colorado traveled with quite an entourage, including filmmaker Becky Popiel, who was there capturing footage for a documentary on Amelia. Michael, Amelia, and I ran Friday evening, and I observed Mike guiding Amelia through the busy midtown streets and Central Park. On Saturday, I ran four miles with Amelia and we talked about race strategy, fueling, and the unique challenges of the NYC Marathon. Amelia was excited and confident. Mike had described her as tough and strong. She was also incredibly open and friendly and expressed an infectious confidence in my guidance and the group assembled to support her run.

We met the Athletes with Disabilities buses in midtown at 5 a.m. Sunday. We drove over the Verrazano Narrows Bridge and were allowed to drop off within a few blocks of the athletes’ village. It was a cold morning with a strong breeze off the water, but they took good care of us with a tent, warm things to drink, and hats. The excitement at the start of the marathon is intense; it is no less intense for athletes with disabilities, but it is different. Amelia did what all athletes do. She focused, prepared methodically, and centered herself. The difference is that we did all that with her. It wasn’t in isolation that she mapped out the race, planned for gel, water, and Gatorade. Crammed in a few more calories. Worried about the bathroom. Wondered about the race.

Except for the hand cyclists who start in front of the field, athletes with disabilities start with everyone else. We were in corral 18, green wave 1. In practice that meant we were at the back of the crowd in the first wave. It’s not a terrible place to start, but it did mean extra bodies to navigate through to try to help Amelia reach her goal. Amelia had three guides at the start. Two of us would run in front and try to clear the road, and one would be at her side. For the first 8 miles or so, Othman Doubiany held the tether as Jamie and I tried to maintain the goal pace and clear the road. We did fine with the pacing, but we were far too polite early on as we tried to make space for Amelia. She fell briefly at mile 7 when we failed to alert her to a timing mat in the road and had a few tangles with other runners who drifted into her space. And each time it felt like a failure and that we had let her down. But Amelia was unflappable. She kept moving and stayed focused on running her race.

Around mile 8, Othman handed me the tether and started using the GoPro he was wearing to get footage of Amelia running. Amelia was running consistently and very confidently—despite our earlier missteps. We barreled through Brooklyn and Queens and fed on the energy of the crowds. Amelia cheered back as folks called out her name and was reveling in the race. As for me, I adjusted to the intense feeling of being tethered to another runner in a very crowded race. I became hyperaware of our surroundings and began to feel comfortable as her guide.

At mile 13, we were joined by two additional guides who had been waiting for us at the Achilles standby table. These two—Angela Riordan and Charles Wilson—are experienced Achilles guides and immediately stepped into the role with enthusiasm. “Blind runner coming through!” “Blind runner behind you on your left!” Along with Jamie, they cleared the course effectively and did an amazing job sprinting ahead to get water and Gatorade. Jamie watched the clock to ensure Amelia was taking a gel every 45 minutes and helped keep the pace in check as the intense wall of cheering hit us as we came off the Queensboro Bridge and into Manhattan.

The long run up First Avenue, miles 16 through 20, is hard. The long straightaway on the way up to the Bronx confronts runners with a little too much time to think about their goals, question their sanity, and wonder if maybe today is not their day. Runners typically hit a wall around mile 18 in a marathon. By mile 20, if they’re going to drop out, they’ve probably done it by now. Amelia was tired and sore, but there was no way she was going to drop out of this race. We hit the Bronx with enthusiasm. She asked us to back the pace off a bit for mile 20—which we did—and to try to pick it up again for mile 21.

Mile 21 in NYC comes as you cross back into Manhattan and can start thinking about home. It’s also a long way from 67th Street and can be emotionally overwhelming: The final six miles of NYC are hilly and difficult. I knew that Amelia was hurting, because everyone is hurting at this point. I had a sharp pain at the top of my right foot that I was starting to think might be a stress fracture. I worried about how we would handle the long climb along Fifth Avenue. And it was hard. For the first time in the race, I could feel resistance on the tether even as we continued to pass other runners. We discussed her goal and how hard to push. For a time, she stressed the importance of her goal and so I did my best to pull her arm in close to mine and hold the 7:45 pace we’d need from that point to make it under 3:30. But soon, as we worked our way up Fifth, it became clear that we needed a different approach.

The crowd offered a wonderful boost though Central Park and we got to the point where I could give her updates every minute or so. Mile 25, just one more rolling hill before we leave the park, the long steady climb up Central Park South, there’s Columbus Circle! 400 meters. 200 meters. There it is. There it is. You’re done, Amelia. You’re done.

As she crossed the finish line, and with me crying pretty uncontrollably, I turned toward her to hug her only to have her collapse into me. Jamie and the others rushed in, supported her, and wrapped her up. They walked with her as we received our medals, snacks, and heat sheets—those metallic “blankets” designed to block out the winds and keep in body heat. We walked with Amelia out of the park and into the athletes with disabilities family reunion area at 72nd Street, where we brought her to the medical tent to try to warm her up a bit.

Running alongside Amelia, intensely watching the road and the racers all around us, listening over the roar of the crowds for any instruction or question, running for Amelia—it became one of the most emotionally overwhelming and wonderful experiences I’ve ever had. Going into it, I wanted to support a woman who loved to run as I love to run. Who loves it so much that the difficulty of running blind among 48,000 others is not going to stop her from doing it. Who is open enough to let us into her life and is willing and able to trust us with her well-being. And all that happened. But on Sunday, running took on an entirely unknown dimension and Amelia’s marathon showed me that running for someone is the best statement about the joy of running I could ever hope to make.

In the end, Amelia Dickerson finished the 2013 NYC Marathon in 3:35:44. She was 6,287th out of 48,000 runners and the 925th woman to finish. I also finished in that time, but I do not appear in the race results. I wasn’t running for me. I was running for Amelia.