When I opened the email telling me I’d been accepted to run the London Marathon, I felt elated. And then terrified. Barely six months on from my last marathon, I knew how dedicated I’d have to be to keep running day after day, week after week, month after month, through rain, cold, tiredness, grumpiness, and hangovers. 

What no one warns you is that the marathon is the easy part. It’s the constant grind of the training that kills you—and finding ways to keep it fresh and interesting is part of the challenge. 

Some exercise nuts think they’ve found a way to do that: by using the AI chatbot ChatGPT as a sort of proxy personal trainer. Created by OpenAI, it can be coaxed to churn out everything from love poems to legal documents. Now these athletes are using it to make all the relentless running more fun. Some entrepreneurs are even packaging up ChatGPT fitness plans and selling them. 

Its appeal is obvious. ChatGPT answers questions in seconds, saving the need to sift through tons of information. You can ask follow-up questions, too, to get a more detailed and personalized answer. Its chatty tone is ideal for dispensing fitness advice, and the information is presented clearly. OpenAI is tight-lipped about the details, but we know ChatGPT was trained on data drawn from crawling websites, Wikipedia entries, and archived books so it can seem to be pretty good at answering general questions (although there’s no guarantee that those answers are correct.)

So, is ChatGPT the future of how we work out? Or is it just a confident bullshitter?

Work it out

To test GPT’s ability to create fitness regimes, I asked it to write me a 16-week marathon training plan. But it was soon clear that this wasn’t going to work. If you want to train for a marathon properly, you need to gradually increase the distances you run each week. The received wisdom is that your longest run needs to be around the 20-mile mark. ChatGPT suggested a maximum of 10 miles. I shudder to imagine how I’d cope if I ran a marathon that underprepared. I’d be in a whole world of pain—and at serious risk of injuring myself. 

When I asked it the same prompt again in a separate conversation—“Write me a 16-week marathon training plan”—it suggested running 19 miles the day before th...