I believe you are right on. It takes a lot longer to recover when older. That's the biggest change I've noticed in my fitness at 69. It's happened in the last couple of years. It's therefore now easier to over-train. I've changed my weekly exercise program to an 8-day week and I do much better.

The modern sports world, at least the pros, have replaced much of the grinding, long-duration training with interval training. I find that periods above 80% and even 90% of my max heart rate (measured) have high payoff and I recover quickly from them. You apparently also get the benefit of raising your lactate threshold which means that you do faster rides at lower heart rates.

Al