Hey Jason, I'm 22 yrs old as well, and had my aortic and mitral valves replaced last year (September, '05). I pretty much felt the same way, a little anxious and a little excited to get it over with. To be honest, I was kinda looking forward to it, because I knew I needed the operation and it was going to be for the best.
And soon your heart will be all fixed up and you'll be riding again! In the coming days, I would say just follow all the instructions your doctor gives and take care of any last minute preparations, maybe spend some time doing something fun, quality time with your family, friends, etc.
A couple tips: take it slow immediately following your surgery, don't try to do too much while you're in the hospital, but at the same time, try to make progress every day.
For instance, about 16 hours after my surgery, I was up walking around the cardiac floor (with a walker and help from a nurse) for maybe 5 minutes. I didn't have to do it, but I wanted to. Next day, I was up walking again without the walker, but still very, very slowly. In the 5 days I spent there, I kept doing it, and sure enough I got stronger.
Following your surgery, continue to cooperate with your doctor/nurses and definitely ask them as many questions as you can -- maybe even write them down so you won't forget -- however, before you leave the hospital, they'll give you plenty of brochures, pamphlets, instructions and guidelines to follow. Oh yeah, and keep your heart pillow handy -- it'll help to buffer some of the pain if you cough or sneeze.
I wish you the best with your surgery, I'm sure you'll come through wonderfully, and your heart will be good as new!