I know what the Regulations say, but all laws come with guidance on how they should be interpreted and applied, which over time are changed through case law, or other means. And indeed the law itself may be amended.
Laws are deliberately written with some areas of flexibility as it is realised that they cannot anticipate every nuance of how they might be applied. They are therefore applied as a combination of the "letter of the law" and the guidance on how that letter should be applied (which can come from a range of sources).
The Government still seems to be making it clear that a "reasonable excuse" for being out of your house is "exercising" (Regulations) for around an hour (informal government guidance), and as a time period is not defined in the Regulations then this is the best guidance we have on this.
But as it's so blindingly obvious that "in general" there are likely to be greater risks of spreading the virus the longer we spend out and about, an hour seems a "reasonable" period of time for most people, but there will be exceptions to this, and I can see why they might not want to specify a time within the Regulations. But, I'm not sure I understand why it doesn't seem to be written down as formal guidance anywhere, at least not that I can find. Maybe the Government is just expecting the public to be sensible about it.
There will be people where their particular circumstances dictate that an hour is insufficient, and they should be allowed to use those circumstances as a "reasonable excuse" to be out for longer than an hour without the stigma of "breaking the law".