I used to do a couple 100 mile rides every year. Usually back to back. There’s a weird psychological effect. If my plan was 100 miles, I’d start getting tired around 85 miles. If my plan was 85 miles, I’d get tired around 60.
Once I missed a turn and ended up doing the 75 mile trip. Because I had planned for 100, I never felt tired.
One of my brothers does a birthday ride every year, a mile per year of age. He turned 73 a few weeks ago. I haven’t asked him where in the ride he started getting tired.