“Now, look,” Tanya said. “If he makes it, you can’t just stick a ribbon on his tail and leave him standing in a field like Eeyore. He’s been abused and abandoned, and that can make an animal crazy with despair. You need to give this animal a purpose. You need to find him a job.”

Secretly, I already had something in mind. It was too ridiculous to say out loud, not unless I wanted to reveal I knew nothing about donkeys and probably shouldn’t have one. It popped into my head as soon as Tanya began assessing the grim wreckage of Sherman’s body, and maybe that’s why I kept circling back to it: Focusing on a glorious long shot was a lot more pleasant than the ugly reality that was kicking us in the face.

So while Sherman was struggling to walk, I was imagining that he could run.

After all, it had worked for me. A few years earlier, I’d been a broken-down ex-athlete battling constant injuries and 50 excess pounds. I hated the monotony of the gym, and the endless yawning miles of cycling. I kind of liked running, but every time I got some momentum, I got hurt. “No surprise,” doctors kept telling me. “Running is terrible for the body, especially big ones like yours.”

For years I believed them — until, in 2006, I found myself in a bizarre adventure at the bottom of a Mexican canyon, where I learned the lost secrets of the world’s greatest ultrarunners: the Tarahumara Indians. I shared those discoveries in my book, “Born to Run,” which became a sensation because so many other people were struggling with the same challenges. Since then, I’ve run thousands of miles in bare feet or the thinnest of sandals, and become convinced that we evolved to fly across the landscape on our own two springy, remarkably durable legs. Movement is our best medicine — so wouldn’t that also be true for Sherman, with the blood of wild African asses in his veins?

I knew just the thing — maybe. While researching “Born to Run,” I’d stumbled across a ragtag crew in the Rocky Mountains who kept alive an old miners’ tradition of running alongside donkeys in races as long as 30 miles. Was it possible? Could I bring Sherman back from this calamity so that he and I, side by side, could run an ultramarathon?

Secretly, I loved the idea of exploring another lost skill the way I had with barefoot running. Animal alliances were once our great art; for most of human existence we relied on other creatures. We could persuade horses and elephants to carry us into battle, and hawks to kill rabbits and drop them at our feet. We could saddle reindeer and herd geese and yoke yaks. Dogs would leap to us at a whistle and throw their bodies in front of anyone who meant us harm. Animals were our companions and transportation, our security systems and wilderness guides. It was a skill all of us shared because animals were all around us. Only recently have we severed that connection and now, with the surge in interest in therapy dogs, celebrity pet trainers, and equine treatment for everything from Parkinson’s disease to sex addiction to post-traumatic stress disorder, we’re trying to recover what we’ve lost.

Running with Sherman would take that challenge to the next level. It would mean forging a bond with a member of one of the most notoriously stubborn species on earth and training for big miles in nasty weather. But first, we had to keep him alive.