I stop my watch after a pacy five-mile run, exhausted and sweating. My running mate stares at me, bright-eyed.‘Is that it?’ she seems to be saying. She’s ready for more. Despite having legs much shorter than mine, she’s barely broken out of a trot for the past 40 minutes. Meet Luna, my three-year-old cavapoo – a poodle King Charles cavalier cross – who has no idea when to stop.

Luna and I have been running together for two years now. We started slowly, unsure if we’d take to this partnership well. But a mile turned into a parkrun and from there we continued to build. I’ve discovered a whole new layer of running joy with my four-legged friend close to my heels. She’s great company, she shares my love of being outside and the rain doesn’t bother her – the muddier, the better is how she feels. We’re kindred running spirits. I chat to her as we go. She rarely answers back; just the occasional yap if I’m not fast enough up the hills.

Yet I’ve never been sure how far I can actually take her. Eight miles? Ten? Is that too much? I waited, of course, until she was fully grown. To do otherwise would have caused her harm. Then a vet told me I could run with her as long as I stopped when she began to look tired. But this is my problem: she never looks tired. At the end of a five-mile run, she will zoom around me in circles, sniffing out a ball or a stick so that she, at least, can carry on. It’s well and truly exhausting.

So I’ve turned to the experts – DogFit, they’re called. One sunny Saturday, Luna and I head to the Surrey Hills. A DogFit trio with their pack of four bigger dogs harness her up with a contraption that makes her leap into my arms. She’s shaking, unsure what this is. But when I’m strapped up, too, she calms down. DogFit comes from canicross – a sport that started in mainland Europe for dogs that pull sleds in the snow. Running cross-country kept them fit in the summer. Ginetta George and Gail Walker started DogFit a decade ago. Now they have 60 trainers and thousands of dogs and runners all over the UK. The kit they’ve designed comes in three pieces: a belt for the human and a bungee lead and harness for the dog.

Now, Luna isn’t terribly sociable with other dogs; she thinks she’s pretty much human. Her doggy interaction is usually chasing off others trying to get her ball. So I wonder how she’ll react to running in a pack. But as soon as we set off, she gets the knack. She pulls out in front, propelling me along – no more running by my ankles. And this dog wants to move along the trails; her tail is wagging and she hardly looks back as we run on woodland paths. It feels great to be so connected with my dog. ‘How far can I actually run with her, though?’ I ask. It turns out that I’m in the company of dog marathoners. One’s even done an ultra after a ‘couch to ultra’, complete with human, in 18 months. It’s about building up slowly, the others say. Almost any dog will run. But, like us, they need to train. You’ll know when they’ve had enough.

Luna and I started with a 5K, but we’ll go back. As we’re leaving, canicross races are mentioned. My ears prick up. ‘One medal or two?’ I wonder before rushing home to google ‘cavapoo 10K PBs’. I’ll let you know how we do...