Rick,

Yes to all.

75/wk is hardly keeping the legs fliud, it doesn't do anything to build any long term endurance. Sadly/Gladly my early season (Jan thru early Apr) are also not much more - 80 mi avg, because of skiing... But I'm glad to make the trade. Skiing does almost nothing for cycling ...
I don't really notice much improvement in cycling for the 60 - 80 mi rides until I start hittin 150 mi/wk. And if I end up on a 40ish mi hammerfest, if there are any late surges; I get shelled badly.

yes, go harder on the hard days, mix in really easy days as often as you need... Sometimes it takes more than 1 easy day to really recover properly. This past week I needed 3 easy weekdays, but had 2 really hard wkdays mixed in; and this weekend the legs felt great/strong and did 2 really hard 60_ mi days... Not allowing proper recovery is a problem we all have when we have time limits to our riding...

and on the easy days really focus on posture and pedaling. Even after so many years of knowing what poor posture does, I still find myself occassionally locking the elbows, hunching the shoulder and getting choppy on the pedals. Little things make a big diff. on a longer ride.

Unless you're heavily muscled, drop 25+ lbs - will make a big difference. I'm just a about 5' 10.5" these days (shrunk over an inch +). And I've noticed a huge improvement in everything in just getting back to the mid 160's (currently 164) down from 170 lbs. I'm targeting 160 for end Sept. and will expect a substantial improvement in my climbing.

I'd add - don;t ignore the upper body strength (along with the all- important core). One doesn't need to be a hulk, but over the years, from the 20's on, my best cycling condition has always been when I've had solid strength in the arms/shoulders and upper back; especially when climbing and riding at speed in a race or fast group.

A rider carrying some extra weight, who is generally also all-body strong, will do okay on most everything except maybe a long climb. But an overweight, soft rider, will not do well in anything, unless they're genetically, aerobically gifted.

Cycling aside, for the age considerations of you and I, continuing allround best health depends a lot on how much muscle we can maintain. It sesems pretty clear that as we age, and our bones/joints become more brittle; muscle becomes essential in providing the need 'suspension' & shock absorption to prevent injuries many elderly suffer.