A nice wee ride - and report - indeed. I live in the middle so enjoy these roads regularly. But have rekindled my love of the southern half of the country.
Your long loop was very ambitious, more than I'd manage. 300+ a day is OK for me as the stops do burn time.
I can remember several years ago I could easily manage a true 50mph average, but those days are gone.
I'm not calling you a liar but, I recall going from my place to Braemar a number of years ago and absolutely thrashing it all the way, as fast as I possibly could just to see what my average would be and I was somewhat disappointed to find it was 46mph, although I did have to pass through Coupar Angus and Blairgowrie which would've obviously brought the speed down considerably.