My legs are stiff and sore today from the vigorous run yesterday. I used to think that being sore was a bad thing, something that should be avoided, but lately I've been reading that being sore is actually an indication that you're working the muscles hard enough. This is what I've learned: The idea that soreness is the result of a build up of lactic acid actually has no scientific evidence. While lactic acid is a by-product of muscle exertion, it doesn't neccessarily cause soreness, nor is it potentially dangerous to your muscles (unless in large doses, which wouldn't be naturally produced by your body).
I always believed that lactic acid was the reason you got sore because that's what I was told. When I played sports growing up, we were told to drink plenty of water and sports drinks after a game to "wash away" the lactic acid. It certainly is important to drink plenty of hydrating fluids after working out, at any level, but you need those fluids to rehydrate yourself; otherwise, you can have organ failure, and that is far more serious than sore calves.
So what really causes soreness? It's actually the result of tiny tears that occur in your muscle tissue as you exercise. Muscle is built on tearing, literally swelling, and healing itself. In other words, you literally tear apart existing tissue to make new tissue, and that is a painful process. However, it's not a bad thing because it means you are challenging muscles and that's how we grow stronger and faster. In fact, if you fall into a "rut" and you find you no longer perform exercise that does make you sore then, perhaps, you need to cross train in order to surprise your body (your body is amazingly adaptive, and once it gets used to a certain physical activity, you not only slow muscle building processes, but you burn fewer calories -- it's the body's way of making whatever exercise you do more efficient) and work new muscles, or work the same muscles in different ways.
Of course, you have to pay close attention to these aches and pains. Soreness that dissipates in a day or two is normal, but deep throbbing or sharp shooting pain that continues unabated for weeks is something to see a doctor about. I once developed shin splints in high school, and they were the most painful thing I'd ever experienced and ended up taking a leave from my soccer team. Protecting yourself from injury is important, but I guess we should all embrace the soreness every now and then; it means progress.