The saddle, tires and handlebars, and the adjustments thereto, are key to a comfortable ride. For someone in your shoes, I highly recommend going to a LBS, describe what you want, have them show you several bikes, and be fitted to it. As I have gotten older, less aerodynamic and am no longer into racing (road or MTB), I find a wider, cushier saddle and a more upright riding position are important for comfort.