Last fall I had only been riding15-20 miles at a time when I did my ride my age (59) ride. It was really tough, and I was beat afterwards.

Lately, I've been building up gradually, and I did the same ride last week, and it was a piece of cake.

Moral: Build up gradually.

Also, I've drunk the HIIT koolaid, and I think that this stuff is relevant:

The Two Best Ways To Build Endurance As Fast As Possible (Without Destroying Your Body)