Replying to posts from 2018… The OP is 74 now. 
Endurance doesn’t fall off a cliff. It slowly declines. Usually the older person falls off a cliff in some way (sickness, illness, a serious fall), then is bed ridden, and then vo2max falls off a cliff. They often don’t get back to where they were.
Indeed but 4000-5000 miles per year is not exceptional at 68. To say so, is a bit patronising. One thing that has come out is that lifelong endurance athletes tend to have better endurance than the younger athlete 30 or so years their junior who is no many years into the sport.
Just consider that in 2023 the average age for riders in PBP (paris brest paris) was 51 
That is it the average for a ride that us 1200km 12000m of climbing and 84 hour time limit ! approximately 7000 riders participated.
Chapeau for a phrase that made my day!
Oh, and semi-on topic, rode 70 solo miles yesterday on my 70th birthday. My wife thought that was impressive given my open-heart surgery in December; for me it was just a very good day on the bike. FTP is still recovering (and improving) as is VO2max. But I wouldn’t hold out my numbers for anyone as aspirational. 
I agree with your wife. Further, your actions speak louder than words or numbers.
Stay rubber side down
J
Yes I’ve been reading that you decline in a series of downward ramps each preceding by something happening that leads to a period of inactivity. Avoid at all cost hospitals. Or if you are unfortunate to have to be in one get your loved ones to keep you supplied with protein drinks and try if possible to be as mobile as you can during your incarceration.
This is AMAZING for anyone who has had open heart surgery in the last year!!!
And don’t we all hope to continue to have good days on the bike into our 70’s and beyond 
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We might even dream of a national title in our mid 70’s, like Jim Mueller!! Check out this episode of the Successful Athlete’s podcast:
That’s a great and inspiring podcast with Jim Mueller. Definitely give a listen if you haven’t yet.
What that FTP translates into is important. Ive not taken the ramp test but I do GCN workouts as well as Spinervals on my Spin bike(Schwann AC Performance Plus, has power meter)Often at end of Spinervals will be a maximum power set. I’m 71 and after HIIT workout 60 minutes I achieve max power of 650 which I can hold for maybe 15 seconds and heart rate maximum of 150ish. On the road without wind Im typically 18 mph rider cruising and can hit 25-27 on flat when all out.
Can cruise on Spinner ay 80-85% max at 190-210 watts, Im 78kg