Truth is somewhere in between. I'm sure we're nowhere near our potential for our age since we haven't don't much aerobic training for decades so it's all about improving and getting the most out of what's left. We're experienced with hard work: I ran marathons into my early 40's and my wife played field hockey to about the same age. I was hoping to catch my #1 son before he had too much base, but I guess I missed my window. His twin is training for triathlons and would be out of reach. This is the most fun I've had "exercising" in 20 years. Our goal is to be fit enough to enjoy 100K - 100mi rides without too much suffering.