My name is Zhiqi Cai. My cousin introduced this forum to me and I found it awesome. I bought K20D couple years ago but didnt shoot much. I own a 18-135mm and recently want to purchase more lens to improve my skill. I am quite a rookie compare to other users in this forum but I will study hard and catch up with you guys one day.
Welcome to the forums. The K20D and 18-135 will serve you well if you like hiking(or general outdoor shooting). It's a great all weather package. I'd pick up a fast 50mm prime on the cheap, for low light, and just practice with the two for now.
I like the 18-135. Keep track of the shots you like best, and look at the focal length. For instance, I found a lot of mine were 28-35mm range and of course zoomed out to get a close shot. Then you can figure out whether you'll be better served with a prime. IMHO, though, the quality of the 18-135 has been pretty good for me. Once I get a replacement lens hood (I lost mine) I'll use it more.
I've also had good results with an old M50 f/2 kit lens, the colors are great. You may not need a very fast lens after all, depending on the shooting you're doing.