Might I suggest you also post in the 65-85+ thread, you should get more relevant replies there. I’m nearing my 72nd birthday and have been riding and racing seriously since the early 1970s. I now just ride for health and fun, and the most important parameter for me is heart rate. I just modulate my effort so that my HR remains in the 100-130 bpm range. Anything lower provides little cardio benefit and anything higher provides no increased benefit and can be overly stressful. I have no interest in spending more than three hours in the saddle, so I rarely ride over forty miles. Also, about half of the time I ride a fixed gear track bike with a very low gear, so that I can maintain a smooth and high cadence. Riding a long distance should be regarded more as an achievement than a goal.