Note the time in zone found by Seiler in that link in post 7. More like 5% of weekly time at the high end, not 20%. That was the main issue. Also my comment on weight work in the gym. The older we get, the more we need it. Your legs shouldn't be that sore, you should "just" have a tired aerobic system.
If that link didn't work for you, here's the same paper in a different journal:
https://www.academia.edu/32019113/Fr...nsity_Analysis