Try a Specialized Body Geometry saddle. They are made in firm, medium, and soft. For longer rides, the "firm" is the most comfortable. Get a version of the BG that is wide at the rear...even a so-called "women's model" if necessary. Most road saddles for men are too narrow at the back and don't fully support the area around the "sit bones".
Set your bars so the top of the bars is level with the top of the saddle. Don't set your saddle too high. Make sure there is still a distinct bend in your knee when the pedal is a six o'clock. Raise your rear off the saddle at every opportunity...anytime you are not pedaling, raise your rear up half an inch by "locking" one leg down straight. Try to pedal in a standing position for a minute or so out of every ten minutes. A good workout for your legs, and it gets the blood circulating through the areas the saddle was pushing on.
It takes a few days to get used to a new saddle and a new riding position and riding style. After you get these things dialed in, you ought to be able to ride a couple of hours and come back feeling better than when you started the ride.