There's a difference between a day out and a single journey, but no hard and fast rules here.
I'm lucky that MrsW enjoys it as much as I do.
So seven and a half hours to the Highlands, followed by lunch then a reasonable walk is fine. We might grab an hours sleep in the late afternoon. But coming home in the same time is exhausting for some reason.
Last year we had a half hour to Blenheim, then in the hot sun until early afternoon, before driving to North Devon on backroads, and that was tiring but OK.
Last weekend we did a day trip to Norwich for the MOG. That was a 5.30 start. The route there is endless, and boring. Almost three hours but only about 150 miles. Then all day walking about, and two and a half hours back. Now that was tiring!
