I agree with your sentiments Tommy about age and determination. I don't know Dave R history(seasoned cyclist, sportsman), but I think he's in his mid 50s and is winning Cat A on Zwift. I've improved no end last year and particularly this winter, so there is always room for improvement. I dont know how far, maybe limited by not having any younger and middle age sporting inclination. It seems those who were quite serious about their sport whilst younger, especially endurance, build a huge heart lung capacity which you cant develop later in life(ignoring genetics) to the same extent. I'm quite interested how this winter translates into the spring and summer on the road.
How far are you from the 6 mins Tommy?
Maybe a few months of HIT training would do it for you.