The long rest section doesn't exclude the activity from the rest period, though. Unless it interrupts, it doesn't count against. That's a very reasonable house rule to have, as is simply interrupting the long rest because you've adventured for almost an hour, but that's not what is written there.
What is written is that you need at least 8 hours of long rest time, 6 of which have to be sleep. That's it. Then it discusses what resets the timer. At no point does it speak about delays. Now, if you include the light activity time you can end up having to extend past 8 hours by virtue of not having enough time in 8 hours to do light activity + 6 hours of sleep + less than an hour of exertion, but that's the only way as written to end up with a delay.