I have been thinking about that lately too. I just turned 64 last month & had my yearly physical & thankfully all the " numbers" were better then last year when I started cycling. I found that I continuely push myself to ride with my average heart
rate into the the high threshold area.For Sept. I decides to stay as much as possible in the aerobic area. So far I found that I am fresher vat the end of the ride and for the most part surprisingly no decrease in speed or length of ride.