To count as interrupting, the adventuring activity must be an hour or more. That defines what an "interruption" is for the purposes of the game mechanic. Adventuring activity of less than an hour is not an interruption. Look again at the sentence and ask yourself, how is an interruption defined by this sentence?
It is defined as "at least 1 hour of walking, fighting, casting spells, or similar adventuring activity". It is not an "interruption" per RAW, unless is is exactly that. Less than an hour of those things is not a period that interrupts. This is really a straightforward parsing of the text.