I think it is common to have that limit, past which things break down. It was 2.5 hours for quite awhile for me, but I kept doing longer rides, and I eliminated the limit. If you routinely do 5 hours, 2.5 becomes a "so what". A'Jet nailed it though: core strength workouts and nutrition/hydration. Also making sure there are no limiting comfort factors, like the wrong saddle. Those things have worked for me.