When I turned 60 I did a 60km (40 mile) ride. I'd just finished getting my medications adjusted to a proper level for a heart arrythmia condition that developed about 4 months earlier, and had worked up to 25-30 mile weekend rides (15 milers 3x during the week), so I bumped it up to 40 miles and had no problems. At 70 y.o. I think I'll just start smoking and drinking; heck, if I make it that far why not!