I'm gonna suggest a two stage approach. Stage 1: Get your fit down on the Rockhopper. You said your knees and butt hurt. Is the saddle too low? Is your leg almost fully extended when at the bottom of the pedal stroke? It should be close to straight, but slightly bent. Next, saddle position. Is your knee over the pedal spindle when at the 3 o'clock position? Past it? If it's past it, move your saddle back. As for your butt, try tilting the saddle up and down to find a position where you don't go numb. If your butt bones hurt, that's ok when you start out, but they should stop hurting after a couple weeks. If it still hurts, look at getting a new saddle. The LBS can measure the distance between your sit bones and recommend an appropriately sized saddle.

You can also mod your bike w/ slicks and a rigid fork. Makes the bike lighter, faster and more efficient.

Stage 2: After getting your fit down on the Rockhopper and riding yourself into shape, evaluate what you want in your next bike. A grocery hauler? A speed machine? An upright cruiser? Room for fenders and a rack? Riding what you have will inform what you want in your next bike.