I’m an adventure racer – I run, cycle and kayak in a team, non-stop over a period of days. I reckon this makes adventure racing the ultimate test of mind and body.

The basic principles are relatively simple, but logistically complicated: your team, usually made up of three men and one woman, has to get from A to B by bike, on foot, in kayaks, sometimes abseiling, sometimes rock climbing or swimming. The clock starts on day one, and doesn’t stop until you either fall by the wayside or cross the finish line, sometimes more than a week later.

There are endurance races all over the world, all year round as part of the Adventure Racing World Series, but the climax of the year is the world championship in November. This is when the very best teams have to be at the absolute top of their game, for the race is held in some of the world’s most unwelcoming terrains – from desert to snow – and is 600 to 800km (375 to 500 miles) long. This means that even those who achieve the fastest times will have raced for nearly 120 hours, or five days, on only a few hours’ sleep.

In 2014, the world championships took place in Brazil. My team and I were going to be racing for 700km (435 miles). Estimates were that the winners would take about 110 hours, ending at a place called Mompiche at sea level. Even if you didn’t count the additional hazards of venomous spiders and snakes, monsoons, ice, jungle and white water, it was going to be a challenging race.

My team and I met Arthur, who was going to become my dog and my best friend, four days into the race. We had had maybe five hours’ cumulative sleep at that point, and I knew that we didn’t have the strength to race this next bit at our best if we didn’t pause at least for an hour or two. The next leg was such a tough one that we decided we would have an extra dose of protein and carbohydrate, so I warmed up two packs of meatballs with pasta. They came with their own thermal sleeve, so you could generate heat around the packs and end up with something almost like a proper meal. These packs are the equivalent of a five-star dinner in the world of adventure racing.

As we sat down to get the food ready, I noticed out of the corner of my eye an unmoving figure by a pile of bike boxes a few yards away. It was a muddy, battered-looking dog. He was standing perfectly still by a red bike box. He seemed to be waiting for something. People were milling around him, but he looked completely unperturbed by all the activity just inches away from him.

I had seen lots of stray dogs in Ecuador, but had never taken much notice of them. Mostly they were sad-looking creatures, with injured legs and missing ears. They would yap and bite and jump about, or they’d howl, or they’d just collapse in a heap asleep. But I had never seen a dog with such presence, such stillness. He was big, and underneath the mud and dirt I thought he was probably a golden colour. Even at a distance I could see that some of the mud was blood; he had bad wounds as well as dirt on him. But he was so stoical, so dignified, he caught my eye by his very appearance of calm.

As I watched, he turned in my direction and padded forwards a few paces. I could see now that he was looking at me. Still a few yards away, he wasn’t making any kind of fuss; he was just looking. I glanced around at the others. They were focused on their food, on their kit. I looked back at the dog. “You’re in a mess, my friend,” I thought. “You’re not complaining, but you’re in a bad way.” He was looking at me unblinkingly.

I knew nothing about dogs. Never had one, never wanted one, but I could see that this dog was somehow special. It was as if he had some sort of inner calm, as if he knew stuff. I opened the pack of meatballs. They were now warmed and looked meaty and good. I put a spoon into the mass inside, got up and moved towards the dog. He carried on looking at me as I approached. Neither coming nearer nor moving away, just looking at me. I got a little nearer, and I bent down and put a spoonful of the meatballs on the ground in front of him. I decided that one wasn’t enough and added another one in front of the dog.

“There you go,” I said. Finally he stopped looking at me and, bending his big head down to the ground, he wolfed the lot almost in one go. “You were hungry, my friend,” I said to him under my breath.

Closer up, I could see that a lot of the mud was indeed dried blood and he had wounds all over him. And when he looked back up at me, every last drop of gravy eaten, I could see that his ears were in a bad way too. I could smell him, too. It wasn’t a good smell. Then I thought how tough it must be to be a stray dog in this country, dependent on the kindness of strangers. I hoped this dog would be OK. I turned away from him and went back to the others.

It was getting dark, and my team-mate Simon, who hadn’t been feeling good and was in the early stages of dehydration, was making slow progress with his kit, so I went over to help him get organised. I then finished packing every bit of my own stuff, carefully piling the layers into my backpack and doing up the buckles. The others were getting ready to lie down and rest. “Twenty minutes,” I said to my team-mates. “And then we’ve got our biggest test of all.”

Hours later, back on the train, I was leading the march. “Staffan?” I asked my team-mate from the front. “How you doing?” “Not good,” he said. “Knee’s bad. Plus I could do with a kip.” He sounded so sleepy as he spoke. “I’ll take your pack,” I said. I seemed to be feeling stronger and stronger, and anyway, this was kind of a payback for him towing me on the bikes earlier in the race.

We stopped for me to take his pack. And that’s when I saw the shape that had been following us draw near. It was the dog that I had given the meatballs to.

We turned round and set off again. We were still making good progress, and I still felt good even though I was carrying two backpacks. Then the track started to get muddier. And muddier. I decided we would have a pause to fix our boots and our backpacks. This next bit was going to be very hard going, so we needed to have our kit and our boots tightly buckled and bound. There was no sign of any team behind us, so we took a moment to sit on some stones by the side of the trail. Our lamps created a pool of light in front of us as we worked.

Aware of a movement by my side, I looked up. It was the dog. He was just standing there, quite still, looking ahead on to the track. “Hey, doggie,” I said. “Shouldn’t you be going home?”

The dog turned his head and looked up at me briefly and then looked back at the track ahead. It suddenly crossed my mind that perhaps he didn’t have a home. I bent down a bit so I could see into his face. He was surveying the scene through half-shut eyes. As I got closer he looked all around him, everywhere but at me. Almost as if he were embarrassed by my close inspection.

“What’s going on, doggie?” I said. “Are you going to come with us?” Then he looked up at me again, looked me full in the eye. I could see his eyes were amber, and he had a dark line round them. But I could also see just how terrible his wounds looked.

His fur was matted and black. I thought of the disease and infections he probably carried around with him. As if reading my thoughts, he looked up at me again, then blinked and looked away. With one smooth movement, he lay down flat in front of me. He put his head down on his paws as if settling in for a bit of a sleep. “What’s your plan, fella?” I said to him, bending down to him. “We’re going deep into the jungle. It’s going to be tough.” The dog looked up at me again. I could hear the others getting to their feet, and I started to get up myself.

About an hour later we heard a group of athletes approaching from behind. They were one of the “short course” teams, teams that don’t have the full time restrictions of the race and have a short cut to the end. They were Ecuadorians and they looked unbelievably fresh to our eyes as they waded quickly and decisively through the mud towards us. When they caught up with us, they stopped.

They looked at Simon and obviously realised how bad he was. The captain of the team asked us in English, “We’ll help you, yes?” I nodded, not knowing quite what they could do. The captain then took off his backpack and got out a bottle of energy drink. “Here,” he said to me. “You have this for him.” It was not the first time that a fellow racer has been so generous – I was once about to pass out from dehydration in the Utah desert. When the captain of a rival team, Richard Ussher, caught up with us, he gave us the only water he had left. “Here, have that,” he had said. It was a wonderful moment of sportsmanship, and so was this.

Very slowly and gradually we gave Simon the liquid. He seemed to get a little calmer afterwards, and we were able to carry on towing him through the mud; Karen in front, us three behind, all, by this stage, struggling. We could see glimpses of the coastline through the trees; the next TA wasn’t too far now. Finally, we emerged out of the thick vegetation of the jungle. Blinking in the light, we could see some ramshackle houses and another river ahead of us. Slowly we edged nearer to the river, past the houses.

My one thought was to get Simon in the water, to cool him down. A few metres further down the river was a woman with a huge pile of clothes; she was slapping them on the rocks, shaking them and wringing them out. We edged Simon a few metres further up the riverbank and then started to walk into the water, with Arthur following us.

Staffan and I used our hands to pour water over Simon, not to drink, just to cool. He grinned and seemed to feel some kind of benefit. We got him back to the bank and looked at the map, to the accompaniment of the now familiar noise of Arthur drinking. “We need to cross this river. How do we do that?” “Well, she’s got a canoe,” I said, looking at the woman doing her washing. “We could ask her to row us there.” Karen looked like we must be joking. But it truly did seem to me to be the only way.

Slightly hesitantly she went up to the woman, with the three of us and Arthur following behind. Karen must have told the woman how ill Simon was, because I could see her putting her washing carefully back on the rock and heading over to her canoe – a long, simple boat with just enough room for us four and her. We walked over towards her and smiled and nodded and smiled and nodded. She seemed happy to help, and we were just happy to be helped.

We put Simon in the boat first and managed to lie him down flat, and then one by one got in behind him. Arthur had now finished drinking, and had followed us to the canoe.

As we bent over Simon, Arthur gave a little whimper. And then, when Staffan got into the canoe, he started whimpering even more. “What shall we do with Arthur?” I said. “He can swim,” said Staffan. “I’m sure he can swim.” But I could see that Arthur was getting more and more distressed as first Karen and then I followed the others into the canoe. Arthur started trotting in agonised circles as the woman picked up her paddle, got into the canoe and pushed us off. In between trying to nod and smile our thanks to the washerwoman I watched Arthur on the shore. He was now whimpering loudly and his circles were getting bigger and bigger. I looked at him and tried to will him to get in the water and come after us. “Come on, come on,” I found myself muttering under my breath. “You can swim, you MUST be able to swim.”

We were now getting further away, and I was just beginning to despair when Arthur suddenly jumped up on to the rock with our kind boatswoman’s washing on it. Scrabbling around and nearly slipping off it, he somehow managed to push the whole pile of washing back into the river. Poor thanks for the great favour she was doing us, but I felt huge elation as he splashed into the water and started swimming after us. I tried to stay still and calm in the boat, but a lot of me was looking back at Arthur as he struggled after us. He clearly wasn’t happy swimming and he was making such slow progress.

“Come on Arthur,” I found myself shouting, “Come on, you can do it!” We were now halfway to the landing stage. Only another hundred metres to go, and Arthur was still swimming. “Hey boy,” Karen shouted. “Nearly there.”

Soon we were only inches away from the landing stage on the other side of the river. As we helped pull up the boat and lift Simon out, I was aware that Arthur only had three metres to go. He looked absolutely exhausted; he seemed to scarcely know how to swim, so he was expending vast amounts of energy trying to stay afloat and move forwards. Perhaps if he hadn’t had all those meatballs he couldn’t have made it.

Finally he got to the bank and emerged from the water beside us, his wet fur making him look thin and bedraggled. We gave a cheer, but it looked as if his legs could scarcely carry him, and as if to prove it he dropped suddenly to the ground.

Later, Arthur watched closely as I peeled off my mud-encrusted socks. Perhaps he wanted to make sure I wasn’t going anywhere – or perhaps he just wanted to remind me that he was still there just in case there were some more meatballs going.

Arthur by Mikael Lindnord (Two Roads, £14.99). To order a copy for £11.99, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min. p&p of £1.99. Information for orders outside the UK can be found on the website.