We've never stuck rigidly to the 5 minute rule. Partly because we're not really people who stick to the rules, but partly because in 1977, there wasn't information overload like there is now. So we really had no idea how far a puppy should be walked.
I could say our first Pyrenean was probably drastically over-exercised as a pup/adolescent. But she was small, and pretty sound, and she lived to 12 years 6 months, which is not bad for a giant breed. So I dpon't actually think it did her any harm. Our others all got less exercise as pups, because we were a bit more experienced, but we still didn't stick to the 5 minute rule. Our second girl, Rose, needed a lot more exercise than the boys did, right from the start.
To some extent you have to trust your instincts. Be cautious, of course, but really, every walk will be different, and sticking rigidly to the recommended guidelines becomes unrealistic. Weather can play a part, pup's mood, your mood, things you encounter. Don't beat yourself up if your pup needs more. Some of them do, and as long as you're sensible and don't push them, it should be fine.