I'm riding a Giant Cypress SX, just started riding again last March. I feel the same way you do, but I've decided to postpone the purchase of a faster bike until at least a year. Meanwhile I will make inexpensive changes to lighten up the load a bit (including losing the last 10-15 pounds off myself). Others have said in other posts on this subject that when the rider gets stronger and lighter, the bike will go faster, so I've decided to take that approach first. But I am seriously considering new wheels and tires on this bike.

Whatever you do, take care of your knee.