I've seen this mentioned on so many posts and I've seen people clarify it within the post but I've not seen a specific post about it. I thought this could help people who seem confused about what is 'allowed'.
Michael Gove was asked on the Andrew Marr show what he considered to be reasonable exercise. This is his response:
"I would have thought that for most people, a walk of up to an hour, or a run of 30 minutes or a cycle ride of between that, depending on their level of fitness is appropriate."
So it is not a hard and fast rule or law. No one is standing with a stopwatch as it gets to 59 mins. It's guidance and it will vary between individuals as Mr Gove says himself.