Strava Premium puts a "suffer score" on each ride, which is a weighted linear combination of time spent in various HR zones. I've been off and on the bike over recent years, now back on. Over the past couple of rides I have noticed a big uptick in my suffer scores. Physically I feel the same, but I notice that my HR is running hotter, about 10 bpm, on the average, and it shows in the suffer score. For example, I went out today for 40-ish miles with maybe 1800 feet of climbing. I've done this ride a few times; here's a couple efforts for comparison:

Early April
Suffer Score: 114
Avg. speed: 14.7 mph, 2:42 moving time
% in Z3 (143 bpm to 161 bpm): 78%
% in Z4 (162 bpm to 178 bpm): 2%

Today
Suffer Score: 165
Avg. speed: 14.7 mph, 2:39 moving time
% in Z3 (143 bpm to 161 bpm): 54%
% in Z4 (162 bpm to 178 bpm): 35%

I asked my doctor, so I'm not just relying on BF wisdom, but I am still waiting for the reply. This month, so far, I'm at a little over 200 miles total, about 9,200 feet of climbing. Nothing extraordinary. But I'm wondering how much fitness-- not age-- can help increase lactose threshold, heart rate, and so on.