How is the angle of your saddle? Is it possible that it is angled a bit down, and that is causing you to slide forward? As a general rule, your butt is going to find the natural place that it fits on the saddle, but that is partly dependent on angle. In any event, you should not correct for that with height - dropping your saddle is very likely to be what is causing your leg soreness. Different heights emphasize different muscles. Get your saddle height close using one of the easy methods, like putting your heel on the pedal and having your leg almost straight. Fore and aft adjustment involves more complicated factors: it's a combination of fitness, bike handling in corners, and your flexibility relative to how low/aero a position you want. Generally, riders who are more fit tend towards a more forward position. Then, of course, there is the uncomfortable truth that you may need to go through a variety of saddles to find one that works for you.