first off - keep up the good work stap - i wish you good health and happiness till 120.
next - the subject of stamina as something which improves with age, as fine wine, seems to repeat itself, in fact we have discussed this very same topic in a previous thread = "Life after 50?" in my response written 25/04 quote:
"As stapfam wrote - "What you have to find is that particular part of riding where experience counts." I'll add endurance since that is one parameter apprently less affected by age."

This might be an interesting research topic - understanding the physiology of stamina as related to ageing.