Its that "Magic" 70 miles or so that has got you. If you haven't trained enough- not taken on enough fuel in the ride and don't carry a reserve of carbs on the body- then 70 miles seems to be about the limit. Train more- but I wouldn't- Eat more and pedal faster. 6 Hour century? You'll beat that time easy
If you don't then face up to it--You are getting older.
__________________
How long was I in the army? Five foot seven.
Spike Milligan