the 90s bashing here is misguided and ignorant. most of the improvement in US running is due to the realization that speed and stamina are connected, not independent of one another, faster milers now race the distances. more importantly, no US record holders 800-5000m were actually doing high mileage.
secondly, the demographics were at a low point in the 90s. 1:44 8, 3:32 15, 809 steeple /13:00 5 were competitive then and still are. none of these achievements require high mileage.
in the 90's it was a big deal for a sub 4 miler to race the 10000m. only low 3:50 milers approach 27' for 10000m. now we have low 3:50 milers in the steeple, 5000 and 10000m and the times are better.
if you look at ethiopian and kenyan running the biggest difference is not added mileage but added quality running. (and the kenyan men are currently getting stomped on the track for neglecting the speed requirements and replacing it with mega-mileage). The best Kenyan marathoner (Sammy) was nowhere near the highest mileage runner.
modern training is high quality running. also note that US marathoning is more or less at an ebb aside from two runners, both who were sadly overtrained and had to adjust/recover. Hall ran 2:04 (wind aided) on lower mileage than he ran the 2:06. Ritz eventually figured out long mileage buildups were counter productive. Hall and Ritz have serious track backgrounds.
now older guys tend to lose much more on the muscle power side than stamina side. I would advise focusing on health first, getting the most out of your 1h/day and trying to get some speed back.