My record is 80km (~50 miles) but usually my uncomfortable seat is the one that makes stop to the long rides.
That could be a simple matter of not getting your sit bones distance measured and finding a saddle that fits those measurements. Most LBS's can help you get measured or you can do it yourself by placing 1 inch thick styrofoam pad about 12 inches square on a hard chair and sitting on it butt naked, then lean over in your typical riding position and let the bones settle into the styrofoam, after about a few minutes get up and you should see two indentations on the foam, simply measure from the center of one indentation to the center of the other. Of course you may want to make sure no one is home when you do that because that would be sort of awkward to explain what you're doing!
There are lots of good saddles but you need those measurements to find the right width saddle for you.